catch somebody on the hop
catch (one) on the hop
To encounter or try to communicate or work with someone at an inopportune time, such as when they are busy or preoccupied; to catch one off guard. I'll definitely help you with that report later—you just happened to catch me on the hop right now. If Dean was grumpy, you probably just caught him on the hop.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
catch somebody on the ˈhop(British English, informal) find somebody in a situation where they are unprepared: The early start of winter that year caught many farmers on the hop.
Farlex Partner Idioms Dictionary © Farlex 2017