As far as I know, the phrase "firehose of adversity" comes from Paul Czege (
here). He's using it to describe his approach to scene-framing, and contrasting it with what he calls "scene extrapolation" which is typical in much D&D adjudication, CoC adjudication, indeed most mainstream/non-indie RPGing.
I believe that
@Ovinomancer is using the phrase in a similar fashion, though he's describing PbtA/FitD play rather than strongly scene-framed play.
The "firehose of adversity" is not
every outcome is as bad as one might possibly conceive. It's that
every situation, including consequences that are snowballing in from previous situations, put something real at stake for the character. The contrast is not between
the orc hurt me and
the orc maimed me but between
you open the door; the room beyond is empty and
you open the door to the vault; the treasure is gone!; or between
you see signs of Orc raiding parties and
as your eyes follow the Orc tracks to the horizon, you see a smudge of smoke from the distance; it looks like it must be coming from your village; or between
the assassin gets the drop on you - you're hosed, and better roll up a new PC and
the assassin gets the drop on you - what do you do? (Baker makes an especial point of that last one in the DitV rulebook, but I think the same point holds for PbtA).