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).