“close-set growth of shrubs, bushes, trees, etc.; tangled coppice or grove,” late Old English þiccet, from þicce in the sense of “dense, growing close together” (see thick (adj.)) + denominative suffix -et. Absent in Middle English, reappearing early 16c., perhaps a dialectal survival or a re-formation.
Etymology:
https://www.etymonline.com/word/thicket#etymonline_v_10751