1.
thanks; hi, hello
Used alone as a casual all-purpose greeting or expression of thanks. Which one it means depends entirely on the situation.
2.
somehow, for some reason
Indicates uncertainty about a reason.
どうも is a versatile word with three distinct functions: casual greeting/thanks, emphasis, and expressing vague uncertainty.
AS GREETING/THANKS (casual):
Used alone, どうも serves as a casual all-purpose greeting or thanks. It can mean "hi," "thanks," or "sorry" depending on context.
- どうも！: Hi! / Thanks!
- どうも、助かりました: Thanks, that helped.
As emphasizer
Strengthens apologies, thanks, and greetings:
As uncertainty marker
Expresses a vague feeling or impression — "somehow," "for some reason":
- どうも変だ: something seems off
- どうもうまくいかない: somehow it's not going well
- どうも彼は嘘をついているようだ: it seems like he's lying
With a negative predicate
どうも〜ない is a very common frame meaning "just can't (quite)" — the speaker is trying but not succeeding, and cannot say why:
This is the same uncertainty sense above, not a separate meaning.
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どうも alone as a greeting is casual. Combined with ありがとうございます or すみません, it is appropriate in polite contexts.