catch on the hop
if you catch someone on the hop, you do something when they are not ready for it and may not be able to deal with it well
catch somebody on the hop
to do something when someone is not expecting it and is not ready

[TahlilGaran] Dictionary of Contemporary English ▲

catch on the hop
catch (someone) on the hop
if you catch someone on the hop, you do something when they are not ready for it and may not be able to deal with it well.
I'm afraid you've caught me on the hop - I wasn't expecting your call until this afternoon.

If we attack at the very start of the game, we may just catch their defenders on the hop.

[TahlilGaran] English Idioms Dictionary ▲

catch someone on the hop
catch (someone) on the hop
if you catch someone on the hop, you do something when they are not ready for it and may not be able to deal with it well.
I'm afraid you've caught me on the hop - I wasn't expecting your call until this afternoon.

If we attack at the very start of the game, we may just catch their defenders on the hop.

[TahlilGaran] English Idioms Dictionary ▲