Is it colloquial to use a proper noun as an adjective if there isn't a corresponding proper adjective? Going further, is using an acronym/initialism as an adjective okay/formal? I'm trying to prove why a specific sentence bothers me so much, and if it's not technically correct English, that'd be why. I know you can say "the HTML tag" and I can't think of a better way to say that, but my coworker wants to use IRS as an adjective describing different emotions and it just bothers me!
An answer is totally fine with me, but a nice linked source would be awesome. My googling skills are just off today. Thanks!