The Holy Mackerel is a melee weapon for the Scout. It was introduced with the Mann-Conomy update.
The Holy Mackerel works similar to the bat. Every time you hit an opponent with the fish, a message will appear in the kill feed saying how many times you have hit an opponent with the fish in that life. If you kill an opponent with the Mackerel, the kill feed will say "Fish Kill!" instead of the normal kill icon.