From Longman Dictionary of Contemporary Englishcatch somebody on the hopcatch somebody on the hopREADYto do something when someone is not expecting it and is not ready → hop
Examples from the Corpus
catch somebody on the hop• Many politicians have been caught on the hop by a good interviewer.• Yes, I think I caught her on the hop.• The dramatic fall in share prices caught even the experts on the hop.• They catch you on the hop.• Sorry about the mess but you caught me on the hop like.• You caught us on the hop there, ol' buddy. 