There is a song called the Old Man's Courtship, or Old Grey Beard kept wagging in which an old man comes courting a young girl, who is obviously not interested(This version GREY BEARD NEWLY SHAVEN in the DT is missing the whole point of the song, I think).

In one verse, we have

Me mother she told me to give him a glass
Hey Ho I won't have him
I gave him a glass and he fell on his head
And his old grey beard kept wagging

(There is a technical name for this kind of substitution, which I can't remember now - 'suspended rhyme' or something.)

But in the following verse, we have

Me mother she told me to give him some bread
Hey Ho I won't have him
I gave him some bread and he fell on his arse (or ass)
And his old grey beard kept wagging

making a joke of the original euphemism