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1.
Ejaculation in a girl's ass, or reference to performing anal sex with a female.
Did you serve her the Trunk Juice?

I took her home and put some Trunk Juice in her.
by Da Memphis General June 27, 2006
 
2.
Juice (specifically a gallon of Hawaiian Punch) that is purchased and left in the trunk of your car over a period of time for consumption.
On my road trips i never to have to buy anything to drink because i always have my Trunk Juice.
by Sanmon June 27, 2011