"In every single trial our toxic brew killed all the bedbugs. We're batting a thousand!"
"In every single trial our toxic brew failed to kill a single bedbug." (Sighs): "We're batting a thousand."
Longer answer: "Batting a thousand," refers to a player's Batting average; I.E. how often he gets a Hit (baseball) divided by how many times he's been At bat.
If he got a hit 29 times after 96 at-bats, he would have a .30308333 batting average. This gets rounded to .303 for all intents and purposes. Batting averages are always three-numeral figures like this. But in spoken English the decimal point is dropped, and the number is stated as a figure in the hundreds, for example, "Three-oh-three" or "Three-hundred-three."