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》

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

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

Woodson, a well-known proponent of one-on-one basketball, said he was OK with Chandler's remarks, which seemed to catch Anthony off-guard. (Anthony said he'd discuss them internally with Chandler.) But either way, the coach has to find a way to get the team jump-started.

WSJ: Sometimes Home Isn't Sweet

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

This week's cancellations seemed to catch many people at Yale off-guard.

WSJ: Ivy Buried by Blizzard, Yale Cancels Class for Two Days