I pulled the plug, and the sink, a seething mess of soap suds, slowly swirled south until all was down the drain save a single last sud.
Wait, can I do that? A sud?
Not everyone would say I could. Some would seethe with resentment. Others would tell me to sod off. I don’t think I’d get sued, though.
But if I did get sued, I’d win. Because yes, though one seldom sees singular sud today, it is, it would seem, the base of suds.
This sud is of not entirely certain origin, but some things are known and others are conjectured. We know that it has long been used predominantly in the plural, and no surprise; one seldom sees a single sud. We also know that before it named soap bubbles, it named scum – wet muck. And as such it seems related to sodden. (However, sod referring to turf is not related, and of course neither is the British crudity as in sod off.)
Sodden in its turn is the past participle of seethe (see the connection?). We know seethe now as a word for anger, and perhaps we imagine seeping steam, but it was originally the usual word for ‘boil’ (the word boil came from French and gradually took over, as many French terms did in the kitchen; the French word came from Latin bullire, ‘boil’, ‘bubble’). Boiling is bubbling, of course!
I would like to say that this is why there is a restaurant near me called Sud Forno. But I’ll have to burst that bubble: that sud is the Italian word for ‘south’ (and forno is ‘oven’). It doesn’t come from Latin, though; Italian took it from French, and French took it from Old English suþ, source of our modern south. That suþ in turn traces back through Proto-Germanic to Proto-Indo-European to a root meaning ‘sun’ that also produced Latin sol and Greek ἥλιος, helios. So somehow the sun floated north like a solitary sud and then flowed back south to become ‘south’. But at least the restaurant isn’t called Sud for no good reason.
So anyway, there is some basis for saying a single sud. But is it common? No. In fact, you won’t easily find instances of its use anywhere, now or over the course of history. Any attempt to deploy it might land with a thud… or perhaps even get the reader into a lather.