A "dead short" is a colloquialism.
In this case, a dead short has no specific value, in the context of the OP's query. It does, however, carry the implied resistance value of the length of conductor that is used as the load being placed on the tested battery.
Given that a car battery might have a capacity of, say, 500 CCAs (at 12VDC), were this entire source placed across a load of 1 ohm, we would see 6,000 watts of power be expended. To the battery, this is a dead short but, as stated, it is actually 1 ohm.
So, take your pick. I might refer you the defintion of "dead slow"...