From "Public Enemies" by Bryan Burrough:
For some reason, probably because they were low on cash, they decided to rob a Muncie roadhouse, the Bide-a-Wee Tavern, that same night. A few minutes after midnight, Dillinger and a partner walked in, guns drawn, handkerchiefs over their faces, and within minutes backed out of the bar with about $70. On the way out the front door, Dillinger encountered a couple coming in. With a grin he pinched the woman's bottom; when her male friend objected, Dillinger slugged him.
When a guy with a gun pinches your girlfriend's ass...just let it go, man.