I am always thrown by people emailing me about "sir names," but it happens quite a lot. English is full of phrases that people mishear and use, like "damn squid" (damp squib), "for all intensive purposes" (for all intents and purposes), "Nip it in the butt" (nip it in the bud), and I recently heard "give it a world" (give it a whirl). However, "surnames" are not something that people discuss that often (at least using that word). In the USA, we prefer to use the term "last name" or occasionally "family name" when speaking about them, and "surname" is usually only found on official forms, if at all.
It kind of makes sense; calling someone "sir" is very polite, and shows a deference to authority/status. It's formal, and using last names is formal. However, if one is knighted, they are known as Sir Full Name (i.e., Sir Anthony Hopkins), not by their last name (never Sir Hopkins).
The word "surname" comes from the prefix sur- meaning "above" + the word "name." "Sur" is used in some words like "surcharge" (a charge above the normal amount), "surface" (the outermost part, or "face" [side] "above"), "surpass," "surrender," "surreal," etc.
Anyway, there is the origin of the word "surname," if you'd like to call me Sir Norah, you may!