An onomatopoetic Japanese phrase piyo piyo suru {ピヨピヨする} which means to make a chirping or cheeping sound. A sound you would normally expect from a chickidee. It is roughly translatable to the "spring of adolescence", where people are carefree and happy-go-lucky. The most familiar use of this phrase is the [[piyo_piyo_apron]] worn by [[Otonashi_Kyouko]], one of the main characters from [[Maison_Ikkoku]]. It is believed by fans of the series to symbolize some kind of immaturity or refusal to move on in life.
piyo piyo (phrase)