On a whim means acting on a sudden fancy rather than a plan. Bought on a whim.
It excuses the decision by admitting there was no reasoning behind it. Things bought on a whim are what fill charity shops.
1phraseon a whimActing on a sudden fancy rather than a plan.
“They booked the whole trip on a whim on a wet Tuesday.”
on a whim is a phrase.
Practised in Idioms