This phrase is used to intensify the meaning of verbs such as bore or charm or kid or scare or talk. For example, That speech bored the pants off us, or It was a real tornado and scared the pants off me. Playwright Eugene O'Neill used it in Ah, Wilderness! (1933): “I tell you, you scared the pants off him,” and Evelyn Waugh, in A Handful of Dust (1934), had a variation, “She bores my pants off.” [Colloquial; early 1900s] Also see bore to death; beat the pants off.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.