It's also kind of neat that it moved from Old French into Old English into Middle Latin.
1381, from O.Fr. endosser, lit. "to put on back," from en- "put on" + dos "back," from L. dossum, var. of dorsum. Sense of "confirm, approve" (by signing on the back) is first recorded in Eng. 1847. Assimilated in form to M.L. indorsare.
Naturally, dossum reminded me of 'dossier'.
1880, from Fr. dossier "bundle of papers," from dos "back," supposedly because the bundle bore a label on the back, from L. dossum, var. of dorsum "back." Or possibly from resemblance of the bulge in a mass of bundled papers to the curve of a back.
Ironic that one's dossier could have such a profound influence on one's endorsements...