What does สบาย (sabai) mean?

The short answer: สบาย (sà-bai) means comfortable, well, or at ease. It stretches over physical comfort, good health and an unhurried frame of mind, all at once. English needs about five words for that range, which is why sabai gets left untranslated so often.

You will almost never hear it entirely on its own. It arrives attached to something:

The part worth noticing

Look at that third row again. "Not comfortable" is how you say you are ill.

Thai does not reach for a clinical word first. If you have a headache, a fever, or something considerably worse, the ordinary thing to say is that you are not sabai. It is what you tell your colleague, and it is what you tell the doctor. The word covers the whole range and nobody finds that imprecise, because the point being communicated is that you are not right, not the diagnosis.

Which tells you something about the positive version too. Being sabai is not a high bar you clear on a good day. It is the baseline, the state you are supposed to be in, and its absence is the thing worth remarking on.

สบายดีไหม is a real question

English speakers tend to hear สบายดีไหม as the local "how are you", fire back a reflexive "fine", and move on. It is worth slowing down, because it is not quite the same move.

"How are you?" in English is a greeting wearing a question costume. Answering it honestly is mildly aggressive. สบายดีไหม is closer to an actual enquiry, and สบายดี is an actual answer. People do say ไม่สบาย back, and then you are having a conversation about it.

ตามสบาย: the hospitality version

Here is where the word earns its reputation. Someone will say this to you within a week of arriving, probably in a doorway.

ตามสบาย is literally "follow comfort". Do as you please. Take your shoes off, sit where you like, stop performing. That last row adds ใจ (jai), heart or mind, and gives you the emotional sense: not physically comfortable, but no longer worried.

Thai builds a great many feelings this way, by attaching them to ใจ. It is one of the more satisfying patterns in the language once you start noticing it.

สนุก and สะดวก, the other two

People searching for sabai usually turn up สนุก and สะดวก alongside it, because the three get quoted together as a set. They are three ordinary separate words, not a fixed idiom, but the grouping is not arbitrary: between them they describe what makes an arrangement good.

สะดวก is the practical one, and it is everywhere: the 7-Eleven on every corner is literally a "convenient-buy shop". It is also the polite way to decline a plan. ไม่สะดวก, not convenient, is softer than no and everybody understands it as no.

สนุก is fun, and it is applied far more widely than the English word. A job can be sanuk. A lesson can be sanuk. Asking whether something was sanuk is a normal way to ask how it went, which is a reasonable thing to have baked into a language.

The one that is harder than all three

If you like this class of word, the one actually worth your attention is not sabai. It is เกรงใจ, and it has no English equivalent at all.

เกรงใจ is the reluctance to put someone out, strong enough that people will refuse things they want in order to avoid it. ไม่ต้องเกรงใจ, the thing you say to release someone from it, is one of the most genuinely useful sentences in the language, and you will need it long before you need most of the grammar.

So why is the app called Sabai Thai

Because that is the claim. Learning Thai has a reputation for being punishing, mostly earned by the tones and the script, and both of those are front-loaded rather than endless. The name is an argument that the process can be sabai: unhurried, not a slog, something you are fine doing for twenty minutes a day.

Whether we pull that off is a separate question, and the app is free, so you can check.