What is 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