Wiktionary
catch flies
vb. (context idiomatic English) To have one's mouth wide open for a prolonged period.
Usage examples of "catch flies".
They work two or three hours at a time, but not hard, and then they knock off and catch flies.
Dano at the door, ready with the old glad hand, grinning like there was no tomorrow, and collecting bits of information the way frogs catch flies.