sin·ful \ˈsinfəl\adjective Etymology: Middle English, from Old English synfull, from syn sin + -full -ful — more at sin 1.: tainted with or full of sin : wicked, iniquitous < sinful men > 2.: marked by or involving sin < sinful thoughts > < sinful practices > 3.: highly culpable < a sinful waste in a world that lacks a sufficiency of food — Lionel Trilling >