"close-set growth of shrubs, bushes, trees, etc.," late Old English þiccet, from þicce (see thick) + denominative suffix -et. Absent in Middle English, reappearing early 16c., perhaps a dialectal survival or a re-formation.
they
thiamin
thick
thicken
thickening
thicket
thickness
thickset
thief
thieve
thievery