土气 literally 'earth/soil air' — describes clothing, hairstyles, or design that feels dated or country-bumpkin. The shorter slang 土 (tǔ) alone is now extremely common online for the same meaning. Often paired with 老土 (lǎotǔ) for emphasis.
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Casual to mildly negative — fine among friends, but calling someone's appearance 土气 to their face can sting.
