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.
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