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A person who is both socially awkward and stupid (although not clinically so).
The person should be considered beyond what is now the socially unacceptable/ not politically-correct term, "retard."

Also, a leotard or unitard worn under clothes.
Bella was excited for us to meet her new friend, but after a moment of conversation, we found him to be an undertard.
by bamwhamthankyoumaam April 27, 2013
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