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