nail in the coffin
The final kick in the ass before someone dies or gets totally destroyed. It’s like the last straw, but way more brutal.
My teacher gave me a D on my test. That was the last nail in the coffin for my hopes of passing this class.
The dog ate my homework. That was the last nail in the coffin for my sanity.
My mom yelled at me for spilling juice on the floor. That was the last nail in the coffin for my day.