What does しまおう mean?


`～おう` means "let's do something", while てしまう seems to mean completion of an
action (A dictionary of basic japanese grammar). In Nihongo so-matome n3 there
is this example: `今日は飲んじゃおう!` which they translate to `I'm going to have a
drink!`. What exactly does ～てしまおう mean?

I've seen some google results for this question, but they were on reddit and
few other websites I can't access from this network.




The te-form is used for stringing verbs together to combine their meanings:

> 飲[む]{L} + しまう→飲[んで]{LL} + しまう→飲んでしまう

しまう usually means "to complete something", but what it actually means depends
on context. [In an answer to an earlier
question](https://japanese.stackexchange.com/questions/36672/v%E3%81%A6%E3%81%97%E3%81%BE%E3%81%86-%E4%BB%96%E3%81%AE%E4%BD%BF%E3%81%84%E6%96%B9%E3%81%8C%E3%81%82%E3%82%8B),
its combined meaning with 飲む is explained as "drinking without thinking about
consequences". An example of 〜しまう meaning "to complete something" (from
naruto's comment):

> 賞味期限が明日だから、今日飲んでしまおう  
>  _It'll expire soon, so better drink it all today_

The volitional form is used to mean things like will, invitation, doubt or
presumption. Here, しまう is conjugated to the volitional form and because the
sentence starts 「今日は」, you can guess that it means the speaker's will to do
something.

> 飲んでしま[う]{L}→飲んでしま[おう]{LL}

If you view 飲んでしまう as "drink unresponsibly", then 飲んでしまおう will become "I'll
drink unresponsibly/anyway".

For comparison, volitional without 〜しまう:

> 飲[む]{L}→飲[もう]{LL} (I'll drink, let's drink)

In Japanese, te-form + aux is often contracted when it connects to frequently
used auxiliary verbs like 〜しまう or 〜いる. 飲ん[でしま]{LLL}う becomes 飲ん[じゃ]{LL}う and
言って[い]{L}る becomes 言ってる. The contracted form conjugates to the volitional form
just like the longer one.

> 飲ん[でしま]{LL}おう↔飲ん[じゃ]{LL}おう




The te-form is used for stringing verbs together to combine their meanings:

> 飲[む]{L} + しまう→飲[んで]{LL} + しまう→飲んでしまう

しまう usually means "to complete something", but what it actually means depends
on context. [In an answer to an earlier
question](https://japanese.stackexchange.com/questions/36672/v%E3%81%A6%E3%81%97%E3%81%BE%E3%81%86-%E4%BB%96%E3%81%AE%E4%BD%BF%E3%81%84%E6%96%B9%E3%81%8C%E3%81%82%E3%82%8B),
its combined meaning with 飲む is explained as "drinking without thinking about
consequences". An example of 〜しまう meaning "to complete something" (from
naruto's comment):

> 賞味期限が明日だから、今日飲んでしまおう  
>  _It'll expire soon, so better drink it all today_

The volitional form is used to mean things like will, invitation, doubt or
presumption. Here, しまう is conjugated to the volitional form and because the
sentence starts 「今日は」, you can guess that it means the speaker's will to do
something.

> 飲んでしま[う]{L}→飲んでしま[おう]{LL}

If you view 飲んでしまう as "drink unresponsibly", then 飲んでしまおう will become "I'll
drink unresponsibly/anyway".

For comparison, volitional without 〜しまう:

> 飲[む]{L}→飲[もう]{LL} (I'll drink, let's drink)

In Japanese, te-form + aux is often contracted when it connects to frequently
used auxiliary verbs like 〜しまう or 〜いる. 飲ん[でしま]{LLL}う becomes 飲ん[じゃ]{LL}う and
言って[い]{L}る becomes 言ってる. The contracted form conjugates to the volitional form
just like the longer one.

> 飲ん[でしま]{LL}おう↔飲ん[じゃ]{LL}おう

