前功尽弃

qián gōng jìn qì

all earlier effort wasted

Character by character

前qián
earlier

功gōng
work

尽jìn
wholly

弃qì
discarded

现在放弃就前功尽弃了。

If you give up now, all your earlier work will be wasted.

Tone: negative (criticism).

From the Stratagems of the Warring States: like the archer Yang Youji, one tired miss can undo a hundred hits.

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