You certainly didn't find that in a dictionary.
You didn't even spell it right, dimbulb.
Right here, dimbulb. http://www.unwords.com/unword/sarchasm.html
Just freepmail your apology. Oh, but only after you wash your hands, please.
I think it's a play on words - "chasm" like an abyss.
Wow, I explain sarchasm and YOU STILL DON'T GET IT!
Who is the dimbulb now? OMG, you kill me. You are so clueless and don't even know it.