laughingstock; object of ridicule
A set phrase meaning someone or something that becomes the target of laughter or mockery. Often used with になる or にする.
He failed and became the laughingstock of everyone.
そんなことをしたら物笑いの種になるよ。
If you do that, you'll become a laughingstock.
Compound of 物笑い (mockery, ridicule) + の + 種 (seed, source), literally 'seed of ridicule'.