Words that are pluralised in the middle poser

[AllYourBassClefsBelongToUs]

I saw the word 'passersby' today which struck me as unusual. I can't think of another word like this - are there any?

There are a few phrases, usually of foreign, maybe french, origin. Coups d'etat and courts martial are the two I can think of off the top of my head.

Hangers-on would also seem to fit the bill but I'm pretty sure it isn't a "proper" word.

Words like mothers-in-law maybe?

this list has about 860 compound words and none of them are pluralised in the middle. I can't fine another one...

Culs-de-sac.

Runners-up.

courts-martial

menservants

attorneys-general

The (now archaic, even obsolete) listener-in follows this pattern. Does it count as it's hyphenated?

yes! Listeners-in.

Spoonsful

Fathers in law
Mothers in law (god forbid)
Estates general
Courts of Law

Grands-prix (I think!)

We have several "titles general" in the US, so besides attorneys general, we have surgeons general and solicitors general.

aides de camp

Anything which contains with a preposition has to be pluralized in the middle. It's the head noun which is pluralized.

Yes, what Nom said. For example, "lookers-on".

Although people mostly say onlookers, now!

trades union

pains au chocolat