Etymology
1. O.E.
þicce "not thin, dense," from P.Gmc.
*theku-, *thekwia- (cf. O.S.
thikki, O.H.G.
dicchi, Ger.
dick, O.N.
þykkr, O.Fris.
thikke), from PIE
*tegu- "thick" (cf. Gaelic
tiugh). Secondary O.E. sense of "close together" is preserved in thickset and proverbial phrase
thick as thieves (1833). Meaning "stupid" is first recorded 1597. Phrase
thick and thin is in Chaucer (c.1386);
thick-skinned is attested from 1545; in fig. sense from 1602. Verb
thicken is first recorded c.1425 (trans.), 1598 (intrans.); an earlier verb was O.E.
þiccian. To be in
the thick of some action, etc., "to be at the most intense moment" is from 1681, from a M.E. noun sense.
2. From Middle English thikke, from Old English þicce (“thick, dense”), from Proto-West Germanic *þikkwī, from Proto-Germanic *þekuz (“thick”), from Proto-Indo-European *tégus (“thick”).
3. Cognates
4. Cognate with North Frisian sjok, tjok, tjuk, tschok (“thick”), Saterland Frisian tjuk (“thick”), West Frisian dik, tuuk (“thick”), Central Franconian deck (“thick”), Cimbrian dikh, dikhe (“thick”), Dutch dik (“thick”), German dick (“thick”), Luxembourgish déck (“thick”), Yiddish דיק (dik, “thick”), Danish tyk (“thick”), Elfdalian tiokk (“thick”), Faroese tjúkkur (“thick”), Icelandic þykkur (“thick”), Norwegian Bokmål and Norwegian Nynorsk tjukk, tykk (“thick”), Scanian tjykker (“thick”), Swedish tjock (“thick”); also Cornish and Welsh tew (“thick”), Irish tiubh, tiugh (“thick”), Manx çhiu (“thick”), Scottish Gaelic tiugh (“thick”). See: Manx '
çhiu'.