Small, possibly temporary, inconspicuous, luggage containing comfortable clothes that may eventually be needed and is packed in anticipation of being “trapped” in a situation or place that requires wearing formal or uncomfortable clothes. A trap bag may be as simple as a paper bag containing shorts, t-shirt, and flip-flops.
Dude 1: “Dude what’s with the suit and tie?”
Dude 2: “I have to go to church with my grandmother.”
Dude 1: “Ouch.”
Dude 2: “Nah, I packed a trap bag with clothes for later.”