also: みなして
expression
everyone together; all at once
Used adverbially to describe a group acting in unison, often with a nuance of doing something simultaneously or as a collective. Common in casual speech.
See also: して
みんなして同じことを言うから驚いた。
I was surprised because everyone said the same thing at once.
子どもたちがみんなして走り出した。
The children all started running together.
Derived from みんな (everyone) + して (adverbial form of する), literally 'doing as everyone'. The exact historical development is uncertain.