Nerf
Etymology
Named after Nerf, the brand of soft foam toys: to "nerf" a weapon is to make it as harmless as a foam dart. The usage arose in online gaming communities in the 1990s (notably around Ultima Online). Its opposite is to "buff."
verb · /nurf/
To weaken a character, weapon, or ability through a game update.
What it names
It gives a single verb for a designer’s balancing act — deliberately dialing something down — and now describes any quiet downgrade of a formerly powerful thing.
Example
They nerfed the healer class so hard that nobody wants to play it anymore.
Related idea
Contrast planned obsolescence, where makers degrade a product to force repurchase; a nerf is usually about competitive balance, not sales.