[AlexCoventry]

It comes from the expression "fired in anger," which meant using a weapon in the heat of battle, i.e., that the weapon was battle tested.

[andrewem]

I’ve seen people (entirely in computing contexts) say “used in anger” but never knew the derivation.

See eg https://en.m.wiktionary.org/wiki/fire_in_anger#Verb

[boomboomsubban]

Ahh, I thought they harbored some ill will towards the project. Thanks.