Do you mean in this sort of context - "If he so much as forgets to wash the dishes, I will leave him"? This is sort of difficult to explain, but it's a phrase used to show how close something is to changing, but hasn't yet. I don't know if you're familiar with the game Jenga, but it's like that - the tower is very close to falling over, and if someone so much as breathes on it, it will surely topple. I hope this helps - I'm happy to follow up if you are still confused.