cosign v.

in Latin, the form of com- "together, with" in compounds with stems beginning in vowels, h-, and gn-; see com-. Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words (co-worker) and Latin-derived words not beginning with vowels (codependent), including some already having it (co-conspirator).

early 13c., signen, "to make the sign of the cross," from Old English segnian and Old French signier "to make a sign (to someone); to mark," both of them from Latin signare "to set a mark upon, mark out, designate; mark with a stamp; distinguish, adorn;" figuratively "to point out, signify, indicate," from signum "identifying mark, sign" (see sign (n.)).

The sense of "to mark, stamp" is attested from mid-14c.; that of "affix one's name or signature to" is from late 15c. The meaning "communicate by hand signs, make known by significant motion" is recorded from 1700.

Transitive sense in baseball, "engage (a player) by the signing of an agreement" is by 1889. To sign out (transitive) "secure the release of (someone or something) by signing" is attested by 1963, of library books. The intransitive sense of "record one's departure" is recorded by 1951. Related: Signed; signing.

also co-signer, "one who unites with another or others in signing a treaty, etc.," 1946, agent noun from cosign. Earlier in this sense was cosignatory (1851).