What's a 4-Letter Word You Say When You're Frustrated?

I'm addicted to crossword puzzles. I used to cut them out of newspapers (or rip the page out of papers I find a Starbucks) but now I just go online and print them off. During my 14 hour flight to the Philippines I had a stack of about 30 of them. When I came home I only had about 3 or 4 left. My goal is to finish them, but I usually have a handfull of squares left. I always hate the ones where you're only missing one letter but you still can't seem to fill it in. Argh!