First attested in 1529; Borrowed from Latin īnsinuātus, perfect passive participle of īnsinuō (“to push in, creep in, steal in”) (see -ate (verb-forming suffix) and -ate (adjective-forming suffix)), from in- (“in”) + sinus (“a winding, bend, bay, fold, bosom”) -ō (verb-forming suffix). Regular participial usage of the adjective up until Early Modern English.