the.com/neckline

means The edge of a garment's opening around the neck and upper chest, whose shape — scoop, V, boat, halter — sets the whole tone of the outfit.

from A plain compound: 'neck' plus 'line,' describing exactly what it is — the line a collar or bodice traces around the neck. English loves stitching simple nouns together this way (hairline, waistline, coastline), and 'neckline' joined that family as garments got precise enough in cut to need a word for the edge itself, not just the collar or trim sewn onto it.