Words with Friends is a popular game for your smart phone or iPad. I don't know how well these words would go over in Scrabble (probably not at all), but they will work when you play with your iPhone/Andriod-toting friends!

BRR – The way you tell people that it’s super chilly and the way you tell your WWF opponents that you don’t care what they think of you.

CWM – Oh, boy – pronounced “koom,” it’s another name for a “cirque,” which is a bowl-shaped mountain basin often containing a lake.

HMM – Accepted (in addition to “hm”) as a sound of contemplation. When you’re thinking just a wee bit harder, it’s “hmm” instead of “hm.”

NTH – Having the quality of being the last in a series of infinitely increasing or decreasing values. (As in, “the nth degree.”)

There's more, with 5, 6, and even 7 letters in this list at mental_floss! Link