blood in the water

Meanings

1. In a competitive situation, the exhibition of apparent weakness or vulnerability by one party, especially when this leads to a feeling of vulnerability or greater pressure to perform on the part of the weak party, or enhanced expectation of victory by the others.
2. Used other than figuratively or idiomatically: the result of a marine predator's attack.

Etymology

Referring to the emergence of predators such as sharks (and possible feeding frenzy) when blood is spilled in the water.