idiosyncrasy

US /ˌɪd.i.əˈsɪŋ.krə.si/
UK /ˌɪd.i.əˈsɪŋ.krə.si/
1.

a strange or unusual habit, way of behaving, or feature that someone or something has:

:
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She often cracks her knuckles when she's speaking - it's one of her little idiosyncrasies.