Via the Dictionaries of Obscure Sorrows & Fantastic Vocabulary:

• abiblioion: n. the action of movement to books
• adronitis: n. frustration with how long it takes to get to know someone—spending the first few weeks chatting in their psychological entryway, with each subsequent conversation like entering a different anteroom, each a little closer to the center of the house—wishing instead that you could start there and work your way out, exchanging your deepest secrets first, before easing into casualness, until you’ve built up enough mystery over the years to ask them where they’re from, and what they do for a living.
• aimonomia: n. fear that learning the name of something—a bird, a constellation, an attractive stranger—will somehow ruin it, transforming a lucky discovery into a conceptual husk pinned in a glass case, which leaves one less mystery to flutter around your head, trying to get in.
• ambedo: n. a kind of melancholic trance in which you become completely absorbed in vivid sensory details—raindrops skittering down a window, tall trees leaning in the wind, clouds of cream swirling in your coffee—which leads to a dawning awareness of the haunting fragility of life, a mood whose only known cure is the vuvuzela.
• anecdoche: n. a conversation in which everyone is talking but nobody is listening, simply overlaying disconnected words like a game of Scrabble, with each player borrowing bits of other anecdotes as a way to increase their own score, until we all run out of things to say.
• anthrodynia: n. a state of exhaustion with how shitty people can be to each other, typically causing a countervailing sense of affection for things that are sincere but not judgmental, are unabashedly joyful, or just are.
• ellipsism: n. sadness that you’ll never be able to know how history will turn out, that you’ll dutifully pass on the joke of being alive without ever learning the punchline—the name of the beneficiary of all human struggle, the sum of the final payout of every investment ever made in the future—which may not suit your sense of humor anyway and will probably involve how many people it takes to change a lightbulb.
• exulansis: n. the tendency to give up trying to talk about an experience because people are unable to relate to it—whether through envy or pity or simple foreignness—which allows it to drift away from the rest of your life story, until the memory itself feels out of place, almost mythical, wandering restlessly in the fog, no longer even looking for a place to land.
• gnasche: n. the intense desire to bite deeply into the forearm of someone you love.
• gnossienne: n. a moment of awareness that someone you’ve known for years still has a private and mysterious inner life, and somewhere in the hallways of their personality is a door locked from the inside, a stairway leading to a wing of the house that you’ve never fully explored—an unfinished attic that will remain maddeningly unknowable to you, because ultimately neither of you has a map, or a master key, or any way of knowing exactly where you stand.
• hiybbprqag: n. the feeling that everything original has already been done, that the experiment of human culture long ago filled its petri dish and now just feeds on itself, endlessly crossbreeding old clichés into a radioactive ooze of sadness.
• Kafkaesque: adj. characteristic or reminiscent of the oppressive or nightmarish qualities of Franz Kafka’s fictional world.
• kairosclerosis: n. the moment you realize that you’re currently happy — consciously trying to savor the feeling — which prompts your intellect to identify it, pick it apart and put it in context, where it will slowly dissolve until it’s little more than an aftertaste.
• lachesism: n. the desire to be struck by disaster—to survive a plane crash, to lose everything in a fire, to plunge over a waterfall—which would put a kink in the smooth arc of your life, and forge it into something hardened and flexible and sharp, not just a stiff prefabricated beam that barely covers the gap between one end of your life and the other.
• lethobenthos: n. the habit of forgetting how important someone is to you until you see them again in person, making you wish your day would begin with a “previously on” recap of your life’s various plot arcs, and end with “to be continued…” after those will-they-won’t-they cliffhanger episodes that air just before the show goes back into months of repeats.
• moledro: n. a feeling of resonant connection with an author or artist you’ll never meet, who may have lived centuries ago and thousands of miles away but can still get inside your head and leave behind morsels of their experience, like the little piles of stones left by hikers that mark a hidden path through unfamiliar territory.
• nementia: n. the post-distraction effort to recall the reason why you’re feeling particularly anxious or angry or excited, in which you retrace your sequence of thoughts like a kid wandering across the neighborhood gathering the string of a downed kite, which was likely lost in a romantic storm or devoured by that huge carnivorous tree that is Things Your Parents Have Said.
• opia: n. the ambiguous intensity of looking someone in the eye, which can feel simultaneously invasive and vulnerable—their pupils glittering, bottomless and opaque—as if you were peering through a hole in the door of a house, able to tell that there’s someone standing there, but unable to tell if you’re looking in or looking out.
• sonder: n. the realization that each random passerby is living a life as vivid and complex as your own—populated with their own ambitions, friends, routines, worries and inherited craziness—an epic story that continues invisibly around you like an anthill sprawling deep underground, with elaborate passageways to thousands of other lives that you’ll never know existed, in which you might appear only once, as an extra sipping coffee in the background, as a blur of traffic passing on the highway, as a lighted window at dusk.
• waldosia: n. [Brit. wallesia] a condition characterized by scanning faces in a crowd looking for a specific person who would have no reason to be there, which is your brain’s way of checking to see whether they’re still in your life, subconsciously patting its emotional pockets before it leaves for the day
• xeno: n. the smallest measurable unit of human connection, typically exchanged between passing strangers—a flirtatious glance, a sympathetic nod, a shared laugh about some odd coincidence—moments that are fleeting and random but still contain powerful emotional nutrients that can alleviate the symptoms of feeling alone.