An informal and somewhat childish word; compare the more formal 大便(daiben).
While うんこ (unko) is sometimes given as a literal translation of English interjection shit! as an expression of displeasure, this is inaccurate, as unko! sounds more like someone saying “Oh turd!” or “Oh doo-doo!” in English. Rather, the vulgar くそ(kuso, “shit, excrement”) or the less vulgar ちぇ(che, onomatopoeic interjection of displeasure) may be used, while the most common expression of displeasure is しまった(shimatta, “oops, uh-oh”, usually when it is one's own fault).