thunk · verb1
By analogy with past participles ending in "-unk", such as drunk and sunk.
thunk · interjection, verb2
Onomatopoeic.
thunk · noun, verb3
Said by the inventors to be from the irregular jocular past tense of think (see Etymology 1), being coined when they realised that the type of an argument in ALGOL 60 could be predetermined at compile time (with a little compile-time “thought”).
Etymology from Wiktionary, CC BY-SA 4.0.