The lawyer's apparently innocent question was designed to catch the witness off (his) guard.

律师看似不经意的问题是为了使证人措手不及。

《牛津词典》

Caught off-guard, Pope slides out of the Taj Mahal room, careful that his wife doesn't catch a glimpse of her anniversary present-to-be.

Pope没有料到她会来，他从制作泰姬陵模型的小间里快步走出，他不想让老婆在结婚纪念日前就看到这个礼物。

youdao

Something related to your salary or your income or your job might catch you off guard, or surprise you in some way.

你的薪水，收入或工作可能会给你带来惊喜。

youdao

But the digital age can still catch her off guard.

WSJ: Toni Morrison talks to Google about creativity

Lindbergh's not mentioning the Jews was to them a trick and no more, the initiation of a campaign of deceit intended both to shut us up and to catch us off guard.

NPR: Roth Rewrites History with a 'Plot Against America'

Stavrum then tried to catch Robertson off-guard in the 20th minute but the Norwegian curled his effort just wide.

BBC: Saints late winner leaves Dons adrift