9:50 p.m., office lights half off. A colleague closes their laptop, stares at the wall for three seconds and says, to nobody: "…しんど。"
Nobody asks what is wrong. Two people make a small sympathetic noise. Someone hands over a coffee.
That is the entire support system, and it worked.
Originally Kansai dialect (from 心労, roughly "heart-labour"), now used everywhere. Younger speakers often clip it to しんど or turn it into a noun: しんどみ, which is basically "the shindoi-ness of it all".
It spans the whole range from "I ran up some stairs" to "I am not okay". Which is exactly what makes it socially useful in a culture where announcing depression at work is still costly. It is a container that can hold anything from a hangover to a crisis, and nobody is forced to ask which one you meant.
That ambiguity is protective, and it cuts both ways. If a Japanese friend says しんどい in a flat voice about something ordinary, that is sometimes as close as they will get to telling you something is genuinely wrong. The correct response is not analysis. It is そっか ("I see") and then being physically present.
You will also hear it about people: あの人といるとしんどい — "being around them is exhausting". Devastating, deniable, perfect.
English has "I'm fine", which is a lie everyone agrees to accept. Japanese has しんどい, which is a true statement that reveals nothing. It lets you tell the truth about your condition without surrendering any privacy — and once you notice how often it gets used, you will realise you have been mistaking a lot of Japanese stoicism for people simply not saying anything at all.