late 14c., "to draw into a smaller compass, become smaller, shrink" (intransitive); early 15c. "make an agreement, enter into a contract, agree or establish to undertake mutually," from Old French contracter and directly from Latin contractus, past participle of 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)). Related: Contracted; contracting.
Meaning "to acquire as by habit or contagion, become infected with" is from 1590s. Transitive sense of "make narrow, draw together the parts (of something) to cause it to shrink" is from c. 1600. Grammatical sense of "to shorten (a word or syllable) by combining or eliding concurrent elements" is from c. 1600. Transitive sense of "arrange for by contract" is from 1897.