A Wizard's Staff Has A Knob On The End
A Wizard's Staff Has A Knob On The End is a famous drinking song, first mentioned in Wyrd Sisters. Due to the song being built around double entendres, Nanny Ogg has taken a shine to it, and she will happily belt it out to the night sky along with the also less-than-wholesome Hedgehog Song.
A version of the song, performed by Dave Greenslade, is available on the CD Terry Pratchett's From the Discworld.
It is noted that wizards themselves find the song rather confusing, and have been known to defensively ask just what is so funny about their staffs having knobs on the end, or about their insistence on polishing them. For foreigners who may likewise find themselves baffled, it's simple: staff, knob and "polishing the staff/knob" are all vulgar slang terms in British English referring to parts of the phallus, or to the act of male masturbation