Much used like the word 'Chillen', Trunkin' is most commonly used as a way to describe what someone is up to. When someone is trunkin' you know they're a laid back person you likes to chill with their friends.
Mark: "Hey Buddy, what's up?"
Eugine: "I'm just trunkin' man, How about you?"
When you have to ride in the trunk because your driver is under the age of 18.
Trunker: I am really beat up today because I had to go trunkin' Last night.
Trunker: We were celebrating because she just got her license.