It's a derogatory term used to insult Pyros by poking fun at the simple strategy of holding down forward (W), pressing fire (M1), and homing in on each target.
The problem with the term is that sometimes that's the best strategy in some situations. Though it never hurts to include some dodging and back-blasting to back it up.
The finesse often comes in setting up for the W+M1. Ambushes and back burner sneak attacks require great skill but they always end in W+M1.
Update: The major exception to the W+M1 tactic is the many weapon combos attacks that the pryo has at her disposal. Nonetheless, W+M1 will still give someone a decent shot at killing another player.