"Blowing It" at the last minute or "Sucking" when it counts the most.
Derived from the 2013 Super Bowl where the San Francisco 49ers had three opportunities to score the winning touchdown at the five yard line at the end of the game and decided to pass, ruining their one sure shot at the title.
The definition also suggests it can be a term for last minute oral sex.
Person 1: "How did the game go?"
Person 2: "We would have won but the qb decided to 49 it!"
Person 1: "Where is Jim?"
Person 2: "He is probably 49ing before work."