1829, "to recollect," a back-formation from reminiscence. Meaning "indulge in reminiscences" is from 1871. "[S]omewhat colloquial" [OED] and mistrusted by the literary (in the OED's earliest citation for it, reminisce is followed immediately by an aside, "the word shall never enter my vocabulary"). Related: Reminisced. Reminiscing as a verbal noun, "action of remembering," is by 1891.