"agarrar desprevenido" in English

to surprise- to wrong-foot

to cart off- to clench- to clutch- to grab hold of- to grapple- to grip- to pick up- to seize- to bite- to bite- to build up- to catch- to catch- to catch- to catch- to catch out- to cop- to do- to get- to get- to get up- to grab- to nab- to nail- to pinch- to hold- to bust- to jump- to rope in- to catch- to get- to nobble- to take- to take- to take- to take up- to catch out- to get- to catch hold- to grasp- to snag- to gather- to get hold of- to go down with- to stump- to take hold of

taken aback- unprepared- unsuspecting- unwary