A phrase used to let others know they've traded sides, i.e. committed treason. This word developed from the blockbuster motion picture 300.
The acts of treason committed by the hunchback traitor Ephialtes stem from his rejection to join the Spartan Military. His morbid anger leads him to join sides with the Persians, thus trading sides in 300.
This phrase is highly adaptable in modern day sense. Whenever someone you see commits an act of treason, betrayal, or pure stupidity, respond hastily with "I knew you'd trade in 300."
Let's say you call Shotgun and Pit steals it from you anyway. You're too borderline blackout drunk to even want to argue with Pit. So, instead of arguing with him, you simply say..."I knew you'd trade in 300."