catch someone 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 someone on the hopBRITISH
COMMON If a person or event catches you on the hop, you are not prepared for them and so are unable to respond quickly or appropriately. She couldn't afford to be caught on the hop if stock sold out. Adam took the fourth qualifying place in his semi-final and admitted his success had caught him on the hop. Compare with keep someone on the hop.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012