A joke, remark, or incident that is extremely funny and likely to provoke loud laughter or physical amusement (traditionally the gesture of slapping one's thigh).
2.
USUKinformalsarcastic
A remark used ironically to indicate that something is not funny or is unimpressive despite being presented as humorous.
Examples:
EN: I didn't realize the concept of our being married was such a thigh-slapper.
ES: No sabía que el hecho de casarse te pareciese tan divertido.
EN: God blessed/cursed me with a thigh-slapper, And I need an x. L. Condom to package the whole thing.
ES: Dios me bendijo (o maldijo) con un fabrica-orgasmos, y necesito un condón XL para cubrir todo el asunto.