"No-no" is an abbreviation for a
baseball no-hitter — a game in which a
pitcher does not allow a single
base hit in nine or more
innings.
Example: "And with that latest strikeout, Nolan Ryan has a no-no through six!"
The term was coined by former Pittsburgh Pirates pitcher Dock Ellis during his LSD-induced no-hitter in 1970. While his team was hitting in the seventh inning, he told second baseman Dave Cash, "Hey, look, I've got a no-no going!"
Source: http://leda.lycaeum.org/Documents/Dock_Ellis_pitches_a_no-hitter_on_LSD..9169.shtml