Nads is a rough, joking slang word for a man's testicles — the two round glands in the bag of skin between his legs. People say it among friends, often to be funny or a bit rude, especially in sport: a ball in the nads is a ball that hits a man there. It is not a doctor's word and not a polite word. You would not write it in school work, or say it to a teacher, to a parent you are being careful with, or to a stranger. It is a short form of gonads, the more scientific word for the sex glands (in men, the testicles; in women, the ovaries). Speakers who say nads almost always mean the male ones. The tone is casual, spoken, and slightly crude — more a laugh than a lesson.