The act of which an ex, whether it be boyfriend or girlfriend, tries to corner you into a situation using various methods, be it "to chill" or "just to talk," to either force you or make you heavily consider getting back together with them. Usually involves sex, kissing, and/or other forms physical contact in an attempt to re-ignite old feelings and trap you.
Josh: Dude, last night my ex called me to just chill and smoke with her, so I went, and we ended up having sex... and now we're back together. Fuck.
Jake: She just pulled an Ex Trap on you, bro, and you fell for it. You dumbass.