More Is Less

A riddle attributed to British prime minister George Canning, among many others:

A word there is of plural number,
Foe to ease and tranquil slumber.
Any other word you take
And add an S will plural make;
But if you add an S to this,
So strange the metamorphosis,
Plural is plural now no more
And sweet what bitter was before.

This is obviously not true. There are many words in English that do not take an -s plural.

I do know one plural that ends in -s such that when you add another s to it becomes singular. Not sure about the "[f]oe to ease and tranquil slumber" part.

That's where I am.

Are you aslumber????

Is it one of these?

princes/princess
ogres/ogress
cares/caress
bras/brass

needles: foe to tranquil slumber?
needless: bitter to sweet?

Naw.

cares to caress... Yeah, AC, I like that one.

I like that cares/caress. Good work, A C.