late 14c. endosse "alteration," from Old French endosser (12c.), literally "to put on back," from en- "put on" (see en- (1)) + dos "back," from Latin dossum, variant of dorsum.
Sense of "confirm, approve" (by signing on the back) is recorded in English first in 1847. Assimilated 16c. in form to Medieval Latin indorsare. Related: Endorsed; endorsing.