It's a mallet that makes people 'grow bigger', in a land with many flat-chested women. I'm surprised it took this long for the combination to surface! The SNK joke was probably only a matter of time too.
Poor Sukuna is going to have so many youkai beating down her door...
I thought the joke was that other characters were flat-chested, but Sakuya's case was that it was pure absence of chest, resulting in hitting the person than the non-existent chest.
