Is it ensure, insure, or assure?
Old English togædere, from to (see to) + gædere "together" (adv.), apparently a variant of the adverb geador "together," related to gadrian (see gather).
German cognate zusammen substitutes second element with Old High German verbal cognate of English same (Old English also had tosamne "together"). Adjective meaning "self-assured, free of emotional difficulties" is first recorded 1966.