contractor (n.)

1540s, "one who enters into a contract," from Late Latin contractor "one who makes a contract," agent noun from past-participle stem of Latin contrahere "to draw several objects together; draw in, shorten, lessen, abridge," metaphorically "make a bargain, make an agreement," from assimilated form of com "with, together" (see con-) + trahere "to draw" (see tract (n.1)).

From 1680s as "a muscle which contracts a part." Specifically "one who enters into a contract to provide work, services, or goods at a certain price or rate" is from 1724.