Actually, if you want to take what I said seriously, cooling liquids just have a high termal capacity and decent or high termal conductivity as well as being at the liquid stage at the temperature range its supposed to work in: a cooling liquid by itself it does not cool anything, it just absorbs heat from the environment on one side of the circuit, carries that heat somewhere else and releases it to the environment there and after that it circulates back to absorb some more heat.

There are plenty of combustible fluids which fit the criteria and could be used as liquid coolants. Whether it would be wise to use a combustible liquid (worse, one which would burn at a high enough temperature to melt, or at least to soften, steel) that’s an entirelly different matter altogether.

The idea of a cooling liquid that’s combustible is actually the “it’s absolutelly possible” part of my post and not the “stupid” part.