"Unt unt" is more of an onomatopoeia than an actual word. The production of this sound is produced by raising the back portion of the tongue upwards for the first "unt" and then downwards for the second. This sound may sound more nasal-like due to the positioning of the tongue. This sound is made often in response to a statement or question in which the speaker disagrees with.
"Did you see her new outfit?"
"Untunt, what did it look like?
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!