A term made popular by Lady Gaga, in her song Summerboy. A summer boy is a boyfriend that you find while on vacation (usually in the Summer, but can also be in Spring or Winter) that you are not in love with or committed to, and he is the same to you. Generally, he does not end up meeting your friends or family. A summerboy is used mainly for sex. Most of the time, a relationship with a summerboy is an open relationship.
Chorus of Lady Gaga's Summerboy:
Lets get lost you can take me home
Somewhere nice we can be alone
Bikini tops, coming o-o-off
Don't be sad when the sun goes down
You'll wake up and im not around
Ive got to go oh, oh, oh, oh, oh
We'll still have the summer after all
Originated from Lady Gaga's song "Summerboy", a summerboy is usually a male companion during the summer used for hook-ups, a "summer boyfriend".
Girl 1 - I didn't know Brad and you would ever be together!
Girl 2 - Yeah, he's just my summerboy.
I've got my summer-summerboy!