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. A: "Did I catch you on the hop?" B: "Yeah, I'm sorry, but if I don't leave right this second, I'm going to miss the train."
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