Thec phrase "used Y in anger" means "used seriously" or "used for its intended purpose". The origin phrase/trope-namer is "never fired its guns in anger" applied to a naval warship in peacetime to describe the case where it might have fired its guns in practice, but not in combat.

Entering "never fired its guns in anger" in your favorite search engine should give several examples.

Here's one in an article from last week. https://owlcation.com/humanities/What-If-The-USS-Arizona-Nev... - "The USS Arizona never fired her guns in anger."