Consider someone capable of doing something, especially something bad. For example, I wouldn't put it past him to tell a lie or two. This expression uses past in the sense of “beyond.” [Late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.