まだ動くうちに売ってしまわなきゃ。

English Translation

I should sell it while it still runs.

Can you explain the use of しまう in this sentence?
I’m used to it meaning “to do something accidentally” or to express regret for doing something. What is its usage in this sentence?

“shimau” basically means “to do something completely”.

If you do something by accident and regret it, you did something that cannot be undone. The accident happened and now you have to live with it, i.e. “the milk is spilled”.

In this example the item is gone once it’s sold.

@ericaw
I guess you know examples like 私は何てことをやってしまったのだ! (Oh no! What have I done?)

If しまう is used as a compound verb, it means “to (already) have done” (a perfect form).

  • If you say 私はやった, it means “I did it”. If you replace it with 私はやってしまった, the altered sentence means “I have done it, and I am crying over spilt milk”. So, “accidental happening” or “some regret” is the connotation in this case.

  • If you say まだ動くうちに売ろう, it simply means “I will sell it while it still runs”. If you replace it with まだ動くうちに売ってしまおう, the altered sentence is “I should have sold before it’s out of order”. まだ動くうちに売ってしまおう sounds like a tactical exit strategy from a zero-sum game. And しまわなきゃ is just a casual shorten form from しまわなければいけない.

Please note that しまう alone (i.e. not as a compound verb) has slightly different meanings: 1) to return something to the original position such as in a storage; 2) to terminate an ongoing project/business/initiative; and 3) to pay off financial liabilities. All of them are derived from the image of “to clean up and back to square one”. So, this image somehow resonates with “to have done” as a compound verb.