Nun Pregnancy

Listen up, buttercup. Nuns aren't supposed to get pregnant because they take a vow of chastity, meaning they don't do the deed. The joke is saying you'd trick a nun by dressing her up as an altar boy, implying that the priest wouldn't realize it wasn't a boy and, well, bingo! It's playing on stereotypes and the whole no-sex-for-nuns thing. You probably still send letters, ya dingus.