Phrase "How's your father?"

We all know that "How's your father?" is a comical (if cliched) euphemism for sex. I am trying to find out where it originated.

I have looked at many resources and asked lots of people but this particular phrase seems to fox everyone.

Does anyone know please?

how's your father? catchphrase associated with the British music-hall comedian Harry Tate (1872-1940). Apparently, he would exclaim it as a way of changing the subject and in order to get out of a difficult situation. The phrase either subsequently or simultaneously took on a life of its own meaning the same as a 'thingummy' or anything the speaker did not wish to name. From that, in phrases like 'indulging in a spot of how's-your-father', it became a euphemism for sexual activity.
From Dictionary of Catchphrases by Nigel Rees
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how's your father casual copulation.
A male expression, perhaps from an opening conversational gambit. Also used of unmarried pregnancy:
The girl was in the club, knocked up, a bun in the oven -- 'ow's yer father. [Gavin Lyall, _The Conduct of Major Maxim_, 1982]
Frequently as 'a bit of how's your father'.
From _A Dictionary of Euphemisms_ by R.W. Holder