on the spur of the moment; by mere chance
Used when an action or outcome happens impulsively or accidentally, without deep intention. Often appears in set phrases like 物の弾みで.
On the spur of the moment, I ended up telling the secret.
I just bumped into you by accident; it wasn't on purpose.
物 (thing) + の (possessive) + 弾み (momentum, impulse). Literally 'the momentum of things', it came to mean an action taken on impulse or by chance.