The compound adjective 'tight-lipped' is a relatively modern English formation, first appearing in the late 19th century. It combines the Germanic root 'tight,' implying firmness and constriction, with 'lipped,' derived from the Old English word for the fleshy part of the mouth. The literal image is of lips pressed firmly together, preventing speech. This vivid physical metaphor then evolved into its primary modern abstract meaning, describing a person who is resolutely unwilling to speak or divulge information, effectively keeping words "locked" behind sealed lips. The logic moves from a concrete physical description to a psychological state of secrecy or restraint.

Structure: tight (firm, secure) + lipped (having lips) - tight: From Old English þēht, tyht (dense, solid, firm). A Germanic root. In this compound, it functions as an intensifier/adverb meaning "firmly" or "securely." - lipped: From Old English lippa (lip). The suffix -ed turns the noun into an adjective meaning "having or characterized by (a specified kind of) lips."