Old English þicce "not thin, dense," from Proto-Germanic *theku-, *thekwia- (cf. Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (cf. Gaelic tiugh).
Secondary Old English sense of "close together" is preserved in thickset and proverbial phrase thick as thieves (1833). Meaning "stupid" is first recorded 1590s. Phrase thick and thin is in Chaucer (late 14c.); thick-skinned is attested from 1540s; in figurative sense from c.1600. To be in the thick of some action, etc., "to be at the most intense moment" is from 1680s, from a Middle English noun sense.