Alteration influenced by Medieval Latin indorsare of Middle English endosse, from Old French endosser (“to put on back”), from Latin dossum, alternative form of dorsum (“back”), from which also dorsal (“of the back”). That is, the ‘r’ was dropped in Latin dossum, which developed into Old French and then Middle English endosse, and then the ‘r’ was re-introduced into English via the Medieval Latin indorsare, which had retained the ‘r’; at the same time the ‘e’ (French) was changed to ‘i’ (Latin) (in-, rather than en-). Note that the alternative form endorse is now more common, retaining the restored ‘r’ but reverting to the initial ‘e’, rather than the Latinate ‘i’.
indorse (third-person singular simple present indorses, present participle indorsing, simple past and past participle indorsed)