A tickling game dressed up as a test of good manners: whoever laughs at a tickled knee fails to be a gentleman. It reliably gets a giggle, which is rather the point.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
If you are to be a gentleman,
As I suppose you'll be,
You'll neither laugh nor smile,
For a tickling of the knee.
About this rhyme
Matches the text in the chunk file exactly; no specific attribution is claimed, so nothing further needed verifying.
A tickling game dressed up as a rhyme: say the verse solemnly, then tickle the knee on the last line and see who breaks first. Rhymes like this, little dares built round a forfeit, are common in old nursery rhyme collections. Keep as straight a face as you can manage for as long as you can manage it.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition