Investment Bang 

when you link a girl who isn't particularly hot, at the moment, because you can tell she will be hot in a couple of years, because then you can 'cash in' on the investment when she's fucking buff as fuck

or in short

the act of securing sex with a member of the opposite sex who shows potential to be very attractive in future years with a view to securing future action when 'the chedder has matured'

dude....your sister requires an investment bang

Investment Bang by Rob and Rob October 1, 2007

Investment banger 

n. someone who specialises in dating seemingly unattractive people that later blossom into fitties.

"Did you see Dave's missus, she's a 9/10". "She used to be such a munter, though. He's such an investment banger."

Investment banger by doctort123 August 20, 2015