as good as dead; might as well be dead
Fixed expression used to describe someone or something that is in such a bad state that it is practically dead or worthless. Often used hyperbolically.
I'm so exhausted I might as well be dead.
This car's engine is broken; it's as good as dead.
A fixed phrase composed of 死んだ (past tense of 死ぬ, 'to die') + も (inclusive particle) + 同然 ('same as, equivalent to'). The expression is transparent and follows standard grammar.