Originally employed to describe words or phrases that morphed (from their common use as symbols representing sex acts) to become more diversely applicable.
The original common use of the terms can be found in older translations of the Bible; hence, the slang.
"know" employed sexually in the Bible: (Gen 4:1; Matt. 1:25)
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Bill: I was 'with' Susie last night...
John: Biblically speaking?!