In TF2, Pyroshark is the name given to the Pyro playstyle of using the Neon
Annihilator, and
hanging out in places like the sewers on 2Fort, the middle section of Well, or near the middle point on
Powerhouse.
This is because the Neon Annihilator crits target that are wet (In or just exited water) so players who use it hang out on maps with water areas exclusively near those areas, hence, Shark.