ないうちに

Meanings

expression

before it becomes; before (something undesirable) occurs

usually written using kana alone

Attaches to the negative stem of a verb (ない-form) to mean 'before something happens' or 'while it is still not the case'. Often used when the following event is undesirable or should be prevented.

暗くらくならない内うちに帰かえりましょう。
Let's go home before it gets dark.
忘わすれない内うちにメモしておいてください。
Please make a note of it before you forget.

Written forms

ない内うちに
Kanji spelling; usually written in kana. The kana form ないうちに is more common.
無ない内うちに
Rare kanji spelling using 無い for ない; not commonly used.

Kanji

内inside, within, between
無nothingness, none, ain't

Similar words

前まえに
前に simply means 'before' and can be used with both positive and negative verbs. ない内に specifically uses the negative form and emphasizes doing something while a state still holds, often to avoid an undesirable outcome.
うちに
うちに means 'while' and is used with affirmative verbs or adjectives. ない内に is its negative counterpart, meaning 'before it becomes' or 'while it is not yet'.

Etymology

Combination of the negative auxiliary ない, the noun 内 (うち, 'inside, within'), and the particle に. Literally 'within the state of not being', it grammaticalized into a conjunction meaning 'before'.