Where do small folks' surnames come from?

Question

So, we know nobles get their surnames from their house (Stark, Greyjoy, etc), which could include commoners with names from fallen houses (Brown Ben Plumm). We know acknowledged bastards get a regional surname (Snow, Waters, Flowers, etc). We know that subsequent offspring of acknowledged bastards might create a hybrid surname (Longwaters, possibly Bywater).

In concept, I could see them coming from a craft like in real life (Miller, Tanner, etc), though I can't think of a character from the books with that kind of name.

What I'm curious about is where do the rest come from? Surnames like Slynt or Deem. They don't seem to fit any of the above patterns, and seems sporadically applied. Any insight into how these names came to be?

Note that the other answer cannot be correct unless someone can explain how Slynt and Deem are Noble (before Janos' elevation) or Bastard names.

Answer 1

The short answer to this is that the small folk do not have surnames. As George R. R. Martin says himself.

Bastard names are given only to bastards with at least one parent of high birth. So the bastard child of two peasants would have no surname at all.
Thus a bastard name like "Snow" or "Rivers" is simultaneously a stigma and a mark of distinction. The whole thing with bastard names is custom, not law.
_{So Spake Martin, SF, Targaryens, Valyria, Sansa, Martells and More}

However, the whole thing is a bit more relaxed than that, as can be seen above with bastard surnames being custom not law. As such it's likely that "rules" around noble surnames are more of a custom and not law though likely held to a higher degree of scrutiny given the weight they carry.

For what it's worth Janos Slynt was definitely a Slynt before becoming Lord of Harrenhall and founding House Slynt. Though he is already the commander of the City Watch at this point.

Lord Renly Baratheon was less sympathetic. "If you cannot keep the king's peace, Janos, perhaps the City Watch should be commanded by someone who can."
Stout, jowly Janos Slynt puffed himself up like an angry frog, his bald pate reddening. "Aegon the Dragon himself could not keep the peace, Lord Renly. I need more men."
_{A Game of Thrones, Eddard VI}

I haven't been able to tie anything down to why this was the case so it could be any one of the following (speculation though of course):

• High ranking members of the City Watch can use names
• Names are custom not law so as long as no one questions it you're fine
• GRRM goofed

As for Allar Deem the same sort of thing is likely considering he was Slynt's "right arm" and so high up again.

"Truly." Tyrion filled Lord Slynt's cup to the brim. "I have been glancing over the names you put forward to take your place as Commander of the City Watch."
"Good men. Fine men. Any of the six will do, but I'd choose Allar Deem. My right arm. Good good man. Loyal. Pick him and you won't be sorry. If he pleases the king."
_{A Clash of Kings, Tyrion II}

Another option for this appears to be that there is a middle class in Westeros that aren't quite Noble yet but are also not common folk. This would appear to match with Slynt and Deem but there is no concrete case for this. It could also be that they just use the last name as a nickname or that no one questions it of course.

---

As for Jeyne Poole, well, House Poole are a noble House of the North.

The largest banners were behind the dais, where the direwolf of Winterfell and the flayed man of the Dreadfort hung back of the bride and groom. The sight of the Stark banner hit Theon harder than he had expected. Wrong, it's wrong, as wrong as her eyes. The arms of House Poole were a blue plate on white, framed by a grey tressure. Those were the arms they should have hung.
_{A Dance with Dragons, The Prince of Winterfell}

Answer 2

Commoners do not have surnames. We don't know the origins of very many House names, but we do know that people without a surname who give themselves one seem to have complete freedom in what they choose.

An example of both of these points can be found in A Dance with Dragons, when Ser Rolly "Duck" Duckfield explains how he got his own surname.

Haldon Halfmaester smiled a thin smile. "Tell our little friend how you came by your name, why don't you?"

"A knight needs more than just the one name," the big man insisted, "and, well, we were in a field when he dubbed me, and I looked up and saw these ducks, so... don't laugh, now."

_{A Dance with Dragons, Tyrion III}

It's possible that the great Houses whose name origins we don't know might've been similarly mundane. It's also possible that people like Janos Slynt are descended from people like Duck, who needed a surname but never established an actual house.