What is a two-word phrase for: acquaintances about whom we know nothing

Question

I need a two-word literary, good-sounding (I know, it sounds vague), and ironic phrase to say something like unknown acquaintances. I want two sides of the phrase to contradict each other. We have an ideal phrase for this in my native language and it sounds something like "people whom we know (acquaintances) whom we don't know." This phrase, however, sounds succinctly as "tanımadığımız tanışlar" where "tanı" means to know/recognize. Now, I am searching for an English alternative of that phrase.

So, here is a bit of context: I am talking about speaker series and I want to invite people whom we meet routinely every single day but don't attempt to get to know them. Examples of such people may be gardeners, dining staff, waiters, police, janitors/janitresses etc.

Answer 1

The technical term for such acquaintances is

consequential strangers

Here is a description from NPR:

You get your coffee from your regular barista. You give a small nod to the guy you see every day at the gym. You don't know these people, but they're vital to your social landscape.

(http://www.npr.org/templates/story/story.php?storyId=112602248)

It's not a very common idiom, but it's an oxymoron, so it does fit your requirement of being ironic or playful.

Otherwise, there are the more common stock phrases:

distant acquaintances

passing acquaintances

nodding acquaintances (implying you know them only well enough to nod at, but not speak to)