Couldn’t find the one I was thinking of, but found this scabrous riddle… :)

My pretty maid, fain would I know,
What thing it is 'twill breed delight,
That strives to stand, that cannot go,
That feeds the mouth that cannot bite.

chorus: With a humble down, humble down, humble down, hey,
Humble down, humble down, humble down, hey.

It is a pretty pricking thing,
A pleasing and a standing thing.
It was the truncheon Mars did use,
A bedward bit that maidens choose.

It is a friar with a bald head,
A staff to beat a cuckold dead.
It is a gun that shoots point blank,
It hits between a maiden's flank.

It is a shaft of Cupid's cut,
'Twill serve to rove, to prick, to butt.
'Twas ne'er a maid but by her will,
Will keep it in her quiver still.

It has a head much like a mole's,
And yet it loves to creep in holes.
The fairest maid that e'er took life,
For love of this became a wife.

http://www.traditionalmusic.co.uk/bawdy-songs/000098.HTM


and another ballad, from Scotland:

http://www.cc.gla.ac.uk/courses/scottish/ballads/mu23y1035.htm