Something or someone that is so useless they serve no purpose. For example tits on a nun serve no purpose, they don't have sex so they are useless a sex objects, they don't have babies so they are useless to produce milk for feeding.

When you're being horny (especially around friends) and as a result shame ensues.

Nuns take a vow of chastity so tits on a nun are useless because you can't feel them.