Reptilious comes from reptile + -ious; reptile from Late Latin reptile ('creeping thing'), from repere ('to creep, crawl'). The historical chain includes Latin reptile and Latin repere. Older glosses in the chain include "creeping thing" and "to creep, crawl", which help explain the semantic range of the modern word. Morphologically, reptilious is best read as compound, suffixal formation, and borrowing, yielding an English adjective shaped by both form history and later usage. In current English it functions as an adjective, with the sense "of or resembling a reptile; coldly cunning or contemptible". related forms include reptile and reptilian. The result is an English adjective whose etymology preserves the older forms while supporting the word's specialized modern meaning.