I'm currently writing an academic report and I began to write out the phrase 'nigh-on-impossible' without a second thought. It then occurred to me that this phrase may actually be slang.
I did a quick Google search and someone on Yahoo answers stated:
Nigh is the Old English word for "near". The phrase means "nearly impossible". There is an alternative: "well nigh impossible" Source
With this in mind, would be it appropriate to use a phrase with such origins in an academic report?