From Wiktionary, the free dictionary
Because such a ball is hit straight at 'em (at them).
at 'em ball (plural at 'em balls)
- (baseball, slang) A hard-hit batted ball hit straight at a fielder in the air for an out.
1996, Mike Whiteford, How to Talk Baseball[1], →ISBN, page 76:If opposing hitters consistently hit balls at -Fielders, the fortunate pitcher is said, to be possessed of an "at 'em ball."