Describes a liquid or mixture that is thin, watery, or lacking body. Often used for soups, sauces, or drinks that are not thick enough.
この味噌汁は分が薄いね。
This miso soup is thin, isn't it?
カレーが分が薄くなってしまった。
The curry ended up watery.
The expression combines 分 (ぶ, 'portion' or 'amount') with 薄い (うすい, 'thin'), literally meaning 'the portion is thin'. It is used idiomatically to describe liquids that are not thick enough.