also: ちびりちびり
adverb, adverb taking the 'to' particle
little by little; bit by bit; sparingly
Mimetic adverb describing doing something in small amounts or gradually, often with a sense of savoring or being careful. Commonly used with drinking or eating.
彼はウイスキーをちびちび飲んでいる。
He is sipping his whiskey little by little.
お小遣いをちびちび使う。
I spend my allowance sparingly.
少しずつ is a more general and neutral way to say 'little by little', while ちびちび carries a mimetic, often colloquial nuance of doing something in tiny, careful increments, especially with consumption.
Mimetic word; the exact origin is uncertain, but it likely derives from the reduplication of ちび (small, tiny) to express doing something in small amounts.