The act of having "put in so many chips", or otherwise risk of consequence, that you might as well follow through with the plan.
Johnny: Y'sure you wanna ditch school early?
Tommy: Well we've made it to Jack in the Box, so I say we're pretty pot committed at this point.
In poker, the act of betting so much that you have effectively put the majority of your chips in play, leaving you with very little left in reserve should you loose the hand.
After being dealt big slick Rob pot committed himself pre-flop and when nothing hit, he was left with a decision of going all-in or checking.