古怪 (gǔ guài) is an adjective meaning 'eccentric', 'bizarre', or 'odd' — describing someone or something that behaves, looks, or feels unusually or unexpectedly different from the norm. The first character 古 (gǔ) literally means 'ancient' and here evokes a sense of old-fashioned, archaic, or out-of-step with modern expectations; the second character 怪 (guài) means 'strange' or 'weird'. Together, they form a compound emphasizing not just strangeness, but a kind of idiosyncratic, slightly unsettling uniqueness — often with neutral or mildly negative connotation, though sometimes affectionate depending on tone and context.

This word commonly describes people’s personalities, habits, or mannerisms (e.g., a reclusive inventor with odd routines), but can also modify objects, ideas, or situations — like a ‘bizarre custom’ or ‘peculiar sound’. It’s more literary and expressive than common alternatives like 奇怪, carrying subtle nuance of enduring, ingrained quirkiness rather than momentary surprise.