Word

かすがい
clamp; cramp (metal); tie (metaphorical, e.g. something that binds two people together)
Word

子は鎹

exp
こはかすがい
children are a bond between husband and wife
Word

豆腐に鎹

豆腐にかすがい
exp
とうふにかすがい
having no effect; waste of effort