expression
the unthinkable; something unexpected; an unlikely thing
Used to refer to something that is hard to imagine or believe, often in negative or hypothetical contexts. The expression is somewhat literary or formal.
彼があらぬことを言い出して、みんな驚いた。
He said something unthinkable, and everyone was surprised.
あらぬことを考えてしまう夜もある。
There are nights when I end up thinking about unlikely things.
Means 'unthinkable' or 'unexpected' as an adjective, while あらぬ事 is a noun phrase referring to the unthinkable thing itself.
The phrase あらぬ事 combines the archaic negative form あらぬ (from ある, 'to be') with 事 ('thing'), literally meaning 'a thing that is not'. The exact historical derivation is uncertain, but it is conventionally used to mean something that does not exist or is beyond expectation.