When you haven't had sex in an incredibly long time or are a virgin that desperately wants to bone and your sexual desire is so fuckin high that you feel like your dick/pussy is gonna explode
Damn man I haven't had sex since 2009, I've got so much pent up sexual energy
Feelings of lust and desire that you can't get off your mind. You don't have to be horny when you are pent-up, but most of the time that's how it goes.
I am so pent-up because I haven't got to spend time with my girlfriend in two weeks.