しっぽをつかむ

尻尾を掴む

to have something on someone; to catch someone out; to trip someone up; to find someone's fault; to get evidence on someone; to get a lead on someone; to grab by the tail

N3Expression
Expression, Godan verb (mu ending)
to have something on someoneto catch someone outto trip someone upto find someone's faultto get evidence on someoneto get a lead on someoneto grab by the tail