偶尔
ǒuěr · HSK 5
once in a blue moon
🌙 偶 = occasional + 尔 = thus/so; together: every so often
偶 carries the sense of accidental or by-chance (also seen in 偶然 by-accident). 尔 is an archaic particle meaning thus or so. The pair reads literally as by-chance-thus, hence once in a blue moon.