Is it ensure, insure, or assure?
mid-14c., from Old French solitude "loneliness" (14c.) and directly from Latin solitudinem (nominative solitudo) "loneliness, a being alone; lonely place, desert, wilderness," from solus "alone" (see sole (adj.)). "Not in common use in English until the 17th c." [OED]
A man can be himself only so long as he is alone; ... if he does not love solitude, he will not love freedom; for it is only when he is alone that he is really free. [Schopenhauer, "The World as Will and Idea," 1818]Solitudinarian "recluse, unsocial person" is recorded from 1690s.