She found out that he’d been fooling around behind her back.
她发现他一直在偷情。
Examples from the Corpus
fool with• This seems like a guy I can fool around with.• I fooled around with a lambchop for a while, but decided I wasn't really very hungry after all.• Why fool around with money, or taking teeth out, when I might not enjoy it as much?• I make a policy of never fooling around with my clients.• Matt thinks his wife is fooling around with someone.• Augie and I are fooling around with them.• Stop fooling around with those scissors before you hurt yourself!