http://en.wikipedia.org/wiki/Jiffy_(time) Jiffy is an informal term for any unspecified short period of time, as in "I'll be back in a jiffy". I won' t be a jiffy. = I won' t be late

I won' t be a jiffy. = I won't be a second. In other words, I'll be quicker than that. http://www.thefreedictionary.com/jiffy

is it, "I won't be but a jiffy?" I won't take a long time. I'll be done very quickly, etc. jiffy = slang for short period of time / quick

sounds very unnatural. is it from a textbook?