Waistline is a compound of waist (from Middle English wast, from a Germanic base related to wax 'to grow') and line. In current use it is a noun meaning "the line of the waist of the body or a garment, or one's girth there". That sense is explained by the older material without requiring a speculative semantic step: the inherited or borrowed base supplies the field of meaning, while the English formation fixes its modern role. The surface form is compositional: its joined elements remain visible in the modern word. The historical chain includes Middle English wast and Latin linea. Related English forms include waist, waistband, and line. The word's history is thus visible in its construction: the components remain interpretable even after the whole expression has settled into a conventional English meaning.