catch someone napping

... a cat-and-dog life争争论吵的日子 catch someone napping乘人不备 catch someone red-handed当场抓获 ...

 catch a weasel asleep

... Her mind was asleep.她的头脑停顿了。 catch a weasel asleep乘人不备 Catch a weasel asleep攻其不备，出奇制胜 ...

 to take advantage of sb in an unguarded moment

to take advantage of sb 揩油 to take advantage of sb in an unguarded moment (idiom) 乘人不备; 乘人不备 to take advantage of sb's misfortune 乘火打劫 ..

 to take sb by surprise

不备 off guard; unprepared 乘人不备 to take sb by surprise; to take advantage of sb in an unguarded moment (idiom) 储备 to store up; reserves ..