catch a Tartar

verb3

1. •
   (informal, no longer used) To discover someone is much stronger, much more dangerous, or much more violent than they appeared at first, especially after laying hands on them

   • Now thou hast got me for a Tartar, To make m 'gainst my will take quarter. — 1674
2. •
   (no longer used) To fail in attempt to rob someone who turns out to be a stronger robber.
3. •
   (figurative, no longer used) To discover someone who cannot be controlled or disposed of more generally.

   • You must give up flirting, my boy, or if I mistake not, you'll find you've caught a Tartar. — 1897