'Thick' is PIE *tegus — one of English's most stable adjectives, unchanged in meaning for 3,000+ years.
With opposite sides or surfaces that are far apart; having a large distance between opposite sides; dense or closely packed.
From Old English 'þicce' (thick, dense, viscous, closely set), from Proto-Germanic *þikuz, meaning 'thick.' The PIE root is *tégus, meaning 'thick, fat.' The word has maintained its core meaning with remarkable stability across three thousand years, shifting only in the relative weight of its senses — from an emphasis on density and closeness in Old English toward a broader range including girth, viscosity, and figurative senses in Modern English. Key roots: *tégus (Proto-Indo-European: "thick, fat").
The phrase 'through thick and thin' originally described riding through a dense forest ('thick') and sparse woodland ('thin') — a literal description of terrain that became a metaphor for enduring all conditions. The phrase is attested from the fifteenth century and appears in Chaucer's Canterbury Tales as 'thurgh thikke and thurgh thenne.'