From the 1811 Dictionary of the Vulgar Tongue, a book for the upper-crust who had no idea what the proles were talking about.
1. Amorous congress
To say two people were engaged in amorous congress was by far the most polite option on the list, oftentimes serving as the definition for other, less discreet synonyms.
"Those two recently opened a basket-making shop." From a method of making children's stockings, in which knitting the heel is called basket-making.
3. Bread and butter
One on top of the other. "Rumor has it he found her bread and butter fashion with the neighbor."
"Yeah, we had a brush once." The emphasis here is on brevity; just a fling, no big deal.
"They left together, so they're probably at clicket." This was originally used only for foxes, but became less specific as more and more phrases for doing it were needed.
Aside from the obvious, this also comes from "making children," because babies have faces.
7. Blanket hornpipe
There is probably no way to use this in seriousness or discreetly, but there you have it.
8. Blow the grounsils
"Grounsils" are foundation timbers, so "on the floor."
See Also: 11 Sexting Acronyms From the 1930s
9. Convivial society
Similar to "amorous congress" in that this was a gentler term suitable for even the noble classes to use, even if they only whispered it.
10. Take a flyer
"Flyers" being shoes, this is "dressed, or without going to bed."
11. Green gown
Giving a girl a green gown can only happen in the grass.
12. Lobster kettle
A woman who sleeps with soldiers coming in at port is said to "make a lobster kettle" of herself.
13. Melting moments
Those shared by "a fat man and woman in amorous congress."
14. Pully hawly
A game at pully hawly is a series of affairs.
15. St. George
In the story of St. George and the Dragon, the dragon reared up from the lake to tower over the saint. "Playing at St. George" casts a woman as the dragon and puts her on top.
16. A stitch
Similar to having a brush, "making a stitch" is a casual affair.
A tiff could be a minor argument or falling-out, as we know it. In the 19th century, it was also a term for eating or drinking between meals, or in this case, a quickie.