c. 1300, "to spread or lay on a surface, to overlay," from Old French couchier "to lay down, place; go to bed, put to bed," from Latin collocare "to lay, place, station, arrange," from assimilated form of com "with, together" (see com-) + locare "to place," from locus "a place" (see locus).
From late 14c. as "to lie down" (intransitive), also "cause to recline upon a bed or other resting place" (transitive). Meaning "lie hidden" is from 1580s. From 1520s as "to put into words;" hence "include the meaning of a word or statement, express in an obscure or veiled way, imply without distinctly saying" (1560s). Related: Couched; couching.
Heraldic couchant ("lying down with the head up") is late 15c., from the French present participle.