An adjective used to describe a foolish or stupid person. It may sometimes be used in a serious context but is generally utilized as a playful insult. It is aslo a "politically correct" way of calling someone a "retard". The word came into useage by way of a lyric in an Ian Erix song and is reportedly an acronym for Un-Non-Tard ("Tard" having been derived from the word "Retard").
Oh my G-d, you've never heard of urbandictionary.com??? You are such an unt!!!
She's the biggest unt...she would lose her head if it wasn't attached to her shoulders!