Typo for moron, which, through its misspelling, comes back at the writer twice as hard as delivered. Esp. in realtime chat-- ie, ICQ. Example: Well, I for one think that Gerri Halliwell is a misunderstood genius. You're a morno. I mean moron.
Meaning of MORNO in English
Slang English vocab. Английский сленговый словарь. 2012