publicum rectumium

Loosely translated (with sincere apologies to Latin scholars everywhere), "people are assholes".

Often said with a resigned tone when something unpleasant has happened to you, and it's viewed as the inevitable consequence of declining politeness and consideration by others.

Roger: Dammit, look at how someone parked me in ... I can't get my car out of the space and I'll be late to meet Fred for lunch.

Tom: Well, Rog, you know what they say ... publicum rectumium