To frighten someone severely (as in 'scare the pants off someone').
To defeat decisively or outperform completely (as in 'beat the pants off someone' or 'beat the pants off an opponent').
2.
generalinformalcolloquial
Used as an intensifier to indicate doing something to excess or with great vigor, often in an athletic or competitive context (e.g., to exhaust, outwork, or outdo).