broken-in

The compound adjective "broken-in" originates from the phrasal verb "to break in," which has been in use since the 16th century with the sense of taming or subduing a horse. The logic is metaphorical: just as a wild animal's spirit is "broken" to make it obedient, a stiff pair of leather boots must have their material "broken" (softened and made pliable) through the wearer's movement "in" them. Over time, this sense of making something new more usable and comfortable through initial, often strenuous, use extended from animals and footwear to clothing, equipment, and even people adapting to new roles. The term perfectly encapsulates the process of transforming something from a rigid, unwieldy state into a serviceable and familiar one.

Structure: broken + in - broken: From Old English brocen, past participle of brecan ("to break"). In this compound, it carries the sense of "having been altered or subdued through force or use." - in: From Old English in ("in, into"). In this compound, it functions as an adverbial particle indicating a state of completion or thoroughness.