'Thin' is PIE *tenh-u- (stretched out) — from *ten- (to stretch). Kin to 'tender,' 'tendon,' and 'tent.'
Having opposite sides or surfaces close together; of small girth; lacking density or substance; lean, not fat.
From Old English 'þynne' (thin, lean, slender, not dense), from Proto-Germanic *þunnuz, meaning 'thin, stretched.' The PIE root is *ténh₂u- meaning 'thin, stretched, drawn out,' from *ten- (to stretch). The original concept was of something drawn out or extended — made thin by stretching. The same PIE root produced Latin 'tenuis' (thin, fine, slender), Sanskrit 'tanú' (thin), and Greek 'tanaos' (stretched out), as well as English 'tender,' 'tendon,' and 'tent.' Key
'Thin' is etymologically related to 'tender,' 'tendon,' 'tent,' 'tension,' and 'tenuous' — all from the PIE root *ten- meaning 'to stretch.' A tent is something stretched over poles; a tendon is a stretched cord of tissue; something tenuous is stretched to breaking; and what is thin has been stretched until its sides are close together.