Otherwise known as a tit job, 31 is the act Of a guy sticking his Penis in between a girls Boobs. It does nothing for you but Guys think its pretty cool and supposedly it feels good.
31 is a person you would expect to do somethign for you, but they won't do it, simply because they do not want to or do not gain anything by doing so and prefer to actselfish for their own good.
You:"Hey man, I am close by could you hook me up, please?"
Other:"No, why should I?"
You:"31"