Is there an idiom that means doing something unilaterally without asking?
I was thinking "on their own accord"?
The airline cancelled my flight on their own accord
Does that make sense? Any other suggestions?
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The word is...
unilaterally (Cambridge Dictionary)
in a way that involves doing or deciding something without first asking or agreeing with another person, group, or country (emphasis mine)
Dictionary example usage...
Employers cannot unilaterally change your contract terms
The airline unilaterally cancelled my flight
In the UK you could say that they did it off their own bat.
idiom UK informal
to do something without anyone else telling you or asking you to do it:
I didn't ask her to buy them a present - she did it off her own bat.
You may hear off their own back meaning the same but it’s only because it sounds similar. Best source I could find is here : https://www.sunderlandecho.com/news/off-your-own-back-and-four-other-common-english-phrases-you-may-be-using-incorrectly-404542
Off your own back
This phrase is often used to refer to something done using one’s own initiative. But in origin it is a cricketing idiom, and should correctly be “off your own bat” – distinguishing runs scored through the batsman’s skill from “extras” accrued without hitting the ball (byes, wides, no-balls, overthrows). This phrase is one of many cricketing idioms in regular use in English.
In the context of the flight cancellation, phrases I might use include "without consulting me" or "without telling me".
"Of their own accord" has the right meaning, but also includes the nuance of "on a whim" or suggests they didn't have the authority to make that decision. While they should have consulted you, an airline does have the authority to cancel a flight.
Not an idiom but a word, quite formal, but commonly used to express the idea of not being told or informed about something.
Unbeknownst / unbeknown to me, the airline cancelled my flight.
Collins Dictionary defines the phrase unbeknown to somebody as “If something happens unbeknown to you or unbeknownst to you, you do not know about it.”
The following would be the equivalent idiom
be kept in the dark (about something)
To be kept uninformed (about something); to be excluded from full knowledge or disclosure (of something).
The OP's suggestion of someone's own accord doesn't quite succeed because it means doing something without being coerced or forced. It expresses the idea of a person's free will; doing something because they want to do it.
- She came of her own accord. No one asked her to come.
- All the demonstrators took part of their own accord.
- He decided to go of his own accord.