"The difference between the right word and the nearly right word is the same as the difference between lightning and the lighting bug." —Mark Twain
I've certainly never seen the word used before and fully never expect to hear it again. On the other hand, I do come across the French expression, "la grasse matinée"[0], (even in an English context) every once in a while and would expect to hear "I enjoyed a nice fat morning in bed" before I ever hear something like "I had a grand old frowst".
Now then, back to my regularly-scheduled frowst.