I know there are a lot of ways to say it, but what is the most commonly used
form?
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2017-04-18T00:51:54.773
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2017-08-01T10:08:39.347
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question
word-choice expressions
What would be the most commonly used and understood term for copulation? (i.e sex)
セックス is the most direct word for it. It's likely not the most scientific way
to describe it, but using セックス in any circumstance would make sense.

In Osaka, I've also heard people (particularly college students) say えっち as
well. However, it's considered slang, and it doesn't quite mean the same
thing.
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2017-05-31T18:34:25.653
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2017-07-31T14:25:38.350
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answer
Depends on context.

「[交尾]{こうび}する」 is, I think, a rather clinical expression, applicable to animals
in scientific literature, etc.

「[性交]{せいこう}する」 is seen a lot, relative to humans, in somewhat formal or
academic contexts.

A lot of the colloquial expressions are borrowed (often mis-borrowed) from
foreign languages. None of them come to mind at once, sorry.

I bought a book called, I think, "Making Out in Japanese", or maybe it was
"Making Out in Kansaiben" once, so I could try to figure out what my wife was
saying sometimes. It didn't really help, which actually was not surprising.

Yet another half-answer, I suppose.
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2017-07-01T09:30:25.287
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answer