The biggest word in the English language is 189,819 letters long, and takes three hours to pronounce! More commonly used big words are several syllables long, and often make people feel smart when they say them out loud. Somewhat ironically, however, study after study has shown that using big words usually makes people sound dumb.
Mark Twain has a couple good quotes about why writers should be economical and precise:
“Don’t use a five-dollar word when a fifty-cent word will do.”
“The difference between the almost right word and the right word is really a large matter – it’s the difference between the lightning bug and the lightning.”
Still, it’s kind of fun to learn that a big juicy ass can be described as “callipygian” and if you’re horny as hell but don’t want anyone to know, just say you’re feeling “concupiscent.”
With that said, below is a list of some of the biggest words in the English language, which you can choose to ignore, or insert into your writing and vocabulary.
1. Abstentious — self-restraining; also the longest word in the English language to use all five vowels in order once
2. Accoutrements — trappings, esp. related to apparel
3. Acumen — ability, skill
4. Anachronistic — a story that didn’t actually happen
5. Anagnorisis — the moment in a story when the main character realizes something that leads to a resolution
6. Anomalist — difficult to classify
7. Antidisestablishmentarianism — a conservative approach to the Church of England in the 1800s; at 28 letters, thought to be the longest non-coined and nontechnical word in the English language
8. Apropos — appropriate
9. Arid — dry
10. Assiduous — painstaking; taking great care through hard work
11. Auspicious — signaling a positive future
12. Behoove — something that is a personal duty
13. Bellwether — the first sheep in a flock, wearing a bell around its neck
14. Callipygian — having large, round, succulent buttocks
15. Circumlocution —the act of using too many words
16. Concupiscent — filled with lust
17. Conviviality — friendliness
18. Coruscant — sparkling
19. Cuddlesome — cuddly
20. Cupidity — greed
21. Cwtch — from the Welsh word for “hiding place”; the longest word in English to be entirely composed of consonants
22. Cynosure — center of attention
23. Deleterious — harmful
24. Desideratum — something needed or wanted
25. Electroencephalographically — relating to an EEG; at 27 letters, the longest word in Merriam-Webster’s Collegiate Dictionary
26. Enervating — exhausting
27. Equanimity — level-headedness
28. Euouae — a medieval musical term; the longest word in a major dictionary entirely composed of vowels
29. Excogitate — to plan
30. Floccinaucinihilipilification — the act of describing something worthless; at 29 letters, the longest nontechnical word to be published in a major dictionary (Oxford English Dictionary)
31. Florid — red and inflamed
32. Fortuitous — lucky
33. Frugal — cheap, thrifty
34. Gasconading — bragging
35. Grandiloquent — verbally pompous
36. Hackneyed — clichéd
37. Honorificabilitudinitatibus — an extremely long-winded way to say “honorable”; at 27 letters, the longest word in the work of William Shakespeare; also the longest word in the English language featuring alternating consonants and vowels
38. Idiosyncratic — peculiar
39. Indubitably — without a doubt
40. Ivoriate — to cover in ivory
41. Lopadotemachoselachogaleokranioleipsanodrimhypotrimmatosilphioparaomelitokatakechymenokichlepikossyphophattoperisteralektryonoptekephalliokigklopeleiolagoiosiraiobaphetraganopterygon — an elaborate fricassee; coined word that appeared in the play Assemblywomen by Aristophanes
42. Methionylthreonylthreonylglutaminylalanyl…isoleucine … the chemical name for titin, the largest known protein; ellipsis used because at 189,819 letters, it’s the largest known word and takes over three hours to pronounce
43. Milieu — environment
44. Nidificate — to build a nest
45. Nonchalant — carefree and unbothered
46. Osculator — one who loves or is loved
47. Paradigm — model
48. Parastratiosphecomyiastratiosphecomyiodes — a species of fly native to Thailand
49. Parsimonious — cheap
50. Penultimate — second to last
51. Perfidious — treacherous
52. Perspicacious — perceptive
53. Pneumonoultramicroscopicsilicovolcanoconiosis —a form of an inflammatory disease; at 45 letters, it’s the longest word to appear in any major dictionary
54. Proficuous — profitable
55. Predilection — preference
56. Pseudopseudohypoparathyroidism — an inherited thyroid disorder
57. Punctilious — meticulous
58. Querulous — fussy
59 Rancorous — bitter and argumentative
60. Remunerative — lucrative
61. Rotavator — a soil tiller; at 9 letters, the longest palindromic word in the English language (i.e., it’s spelled the same way backwards)
62. Saxicolous — something that lives on rocks
63. Sesquipedalian — involving long words, just like this article
64. Splendiferous — wonderful
65. Squirrelled — put away; the longest one-syllable word in the English language
66. Subdermatoglyphic — regarding patterns formed by fingerprints; at 17 letters, the longest word in English where not a single letter is repeated
67. Supercilious — arrogant
68. Synergy — extra energy generated by cooperation
69. Unencumbered — free
70. Unparagoned — without equal
71. Winebibber — an alcoholic 
Source : https://thoughtcatalog.com/jeremy-london/2018/06/big-words/