Non-native speaker is correcting a native speaker, the latter being [edited] It's "wide off".

Hate to break it to you, but it's wide of the mark! A well known English idiom! Perhaps you should check things like this on google before you start pointing fingers in future; it would save you the embarrassment of me correcting you again! Shouldn't an experienced tertiary professor of linguistics know GIS idioms a little better? I do hope you haven't been teaching your poor students "wide off"?! Or is your descriptive grammar now also being applied to idioms?!

Just so we're on the same page and to hopefully avoid the inevitable assertion to the contrary...

http://idioms.thefreedictionary.com/_/dict.aspx?rd=1&word=wide+of+the+mark

I'm really beginning to think I should be charging you for all of these free lessons!