At 'em ball -- A ball hit right at a defensive player.
Baltimore Chop -- A ground ball that hits in front of home plate (or off of it) and takes a large hop over the infielder's head.
Bases drunk -- Bases loaded.
Bazooka -- Strong throwing arm. "He's got a bazooka."
Big show -- The major leagues.
Bush league -- Lesser minor-league teams in small cities or towns.
Catch napping -- To surprise a less than alert runner with the result that he is picked off or suddenly caught between bases.
Chin music -- A beanball or knockdown pitch that passes close to the batter's jaw.
Circus catch -- A spectacular catch, suggesting the moves of a circus acrobat.
Crackerjack -- Term used to describe a first-rate or spectacular player or team.
Curtain call -- The practice of a player coming out of the dugout to acknowledge the call of the fans, usually after a home run.
Deuce -- The curveball, usually signaled from a catcher to a pitcher by holding down two fingers.
Dying quail -- A batted ball that drops suddenly and unexpectedly, like a bird that has been shot on the wing.
Fence buster -- A heavy hitter.
Free pass -- Base on balls.
Friendly confines -- Descriptive of many home ballparks, but most often used to describe Wrigley Field.
Glass arm -- A sore throwing or pitching arm.
Golden Sombrero -- A batter who strikes out four times in the same game is said to wear the golden sombrero.
Goose egg -- A zero on the scoreboard.
Gopher ball -- A pitch destined to be hit for a home run; one that will "go for" a run.
Guess hitter -- Batter who tries to anticipate or out-guess the pitcher based on the situation at hand.
"Hit 'em where they ain't" -- Rallying cry for batters through the decades since 1897, when Wee Willie Keeler hit .432. Asked how a man of his size could put together such an average, Keeler responded: "Simple. I keep my eyes clear and I hit 'em where they ain't."
Hot stove league -- Term for the gab, gossip and debate that takes place during the winter months when baseball is idle.
Iron glove -- Sloppy fielding; prone to errors.
Lollipop -- A soft pitch or weak throw.
Mendoza line -- Figurative boundary in the batting averages between those batters hitting above and below .215, Mario Mendoza's career average.
Picasso -- A control pitcher; one who paints the black.
Platter -- Home plate.
Play by the book -- To play in accord with the conventional wisdom of the game.
Pow wow -- A meeting on the playing field, usually involving several players and a coach who has come on to the field to talk strategy.
Punch-and-Judy -- Said of a hitter who tends to hit well-placed but weakly-hit balls for singles. The first reference was made by former Los Angeles Dodgers manager Walter Alston when asked about a home run by Willie McCovey of the San Francisco Giants. "When he belts a home run, he does it with such authority it seems like an act of God. You can't cry about it. He's not a Punch and Judy belter."
Rhubarb -- A ruckus with the umpires; confusion; a fight between players.
Right down Broadway -- A pitch that is delivered in the middle of the strike zone.
Room service -- Term describing a pitch or batted ball that comes right to a defensive player.
Shoestring catch -- A catch made by a running fielder just before the ball hits the ground.
Texas Leaguer -- A poorly hit ball that loops meekly over the infield and lands for a hit.
The Show -- The major leagues
Tools of ignorance -- The catcher's paraphernalia: shin guards, chest protector, helmet, mask and glove.
Twin killing -- Double play.
Warning track power -- The ability of a batter with enough strength to hit a ball to the warning track, but not enough to hit a home run.
Whiff -- For a pitcher to strike out a batter.
Whitewash -- To shut out a team.
Worm burner -- Batted ball that moves across the ground hard and fast.
Source:Dickinson's Baseball Dictionary