noun
A derogatory term for a person who is physically or mentally weak, lacking courage or resilience. Rare and somewhat dated; not commonly used in modern conversation.
彼は弱みそだから、ちょっとしたことですぐ泣く。
He's such a weakling that he cries at the slightest thing.
弱みそ呼ばわりされて、彼は怒った。
He got angry when he was called a coward.
弱虫 is a more common and modern term for a coward or weakling, while 弱みそ is rarer and carries a slightly old-fashioned or rustic nuance.
Compound of 弱 (yowa, 'weak') and 味噌 (miso, 'miso paste'), possibly from the idea of something soft and easily crushed. The exact origin is uncertain.