The longest word in literature is the fictional dish described in Aristophanes’ comedy Assemblywomen, written in around 391 BC.

λοπαδοτεμαχοσελαχογαλεοκρανιολειψανοδριμυποτριμματοσιλφιοκαραβομελιτοκατακεχυμενοκιχλεπικοσσυφοφαττοπεριστεραλεκτρυονοπτοκεφαλλιοκιγκλοπελειολαγῳοσιραιοβαφητραγανοπτερύγων

which is the name of a dish compounded of all kinds of dainties, fish, flesh, fowl, and sauces.

which, in the Roman alphabet is written as:

Lopado­temacho­selacho­galeo­kranio­leipsano­drim­hypo­trimmato­silphio­parao­melito­katakechy­meno­kichl­epi­kossypho­phatto­perister­alektryon­opte­kephallio­kigklo­peleio­lagoio­siraio­baphe­tragano­pterygo

with a rough English translation of:

oysters-saltfish-skate-sharks’-heads-left-over-vinegar-dressing-laserpitium-leek-with-honey-sauce-thrush-blackbird-pigeon-dove-roast-cock’s-brains-wagtail-cushat-hare-stewed-in-new-wine-gristle-of-veal-pullet’s-wings
Leo Strauss (1966)

The longest non-technical word in English is:

floccinaucinihilipilification

meaning:  the act of estimating something as worthless.