What is How's Your Father?
1.
Slap 'n' tickle; Bit of the other; casual sexual relations; Origin in World War II, English soldiers in France expected that an old French lady with grey hair, whom their father had bonked during the First World War might come up to them and ask this.
"Took her round the bike shed for a bit of how's-your-father";
"I don't want you two getting up to any how's-your-father while we're out!"
2.
Sexual intercourse (English 20th century)
Provenance: 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_ (1995) by Nigel Rees
They were in the snug engaging in a spot of how's your father.
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3.
sexual intercourse or other sexual activity
According to Michael Kelly, a writer and historian in New Zealand, "the origin of the expression 'how's your father' can be traced back to Victorian times. In those days any man with a daughter was so protective of her virtue that he would take extraordinary measures to safeguard it. Unmarried girls would be kept within the bosom of their family as much as possible, chaperoned on excursions, and on those occasions when they were let out of bounds for social events, their fathers would often accompany them discreetly by hiding underneath their voluminous skirts ready to pounce on any man who transgressed the bounds of propriety.
However, a father with more than one daughter couldn't be everywhere at once. Thus, a suitor having a discreet vis-a-vis with his beloved would cautiously ascertain her father's whereabouts by asking, 'And how is your father?' If her father was currently under her skirts, she would glance downwards and reply, 'My father is very well, thank you, and as alert and vigorous as ever, and maintains his interest in rusty castrating implements.' Her beau would then say, 'I have always had the greatest respect for your father, and of course for you. Let us hold hands and think about the Queen for a while.' If, on the other hand, her father was elsewhere, she would reply, 'The mad old bastard is currently stationed between my sister Constance's thighs. Let us go into the garden and rut like stoats.'
Hence, 'How's your father' became a euphemism for you-know-what."
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