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1.
a verb that is used to describe when someone is exhibiting traits commonly found in people with down syndrome. i.e: acting/saying/doing something dumb.
Brendan: My favorite color is peanut butter

Laura: Quit downsing.

alternately

Brendan: Sometimes I smell my wallet to see if it smells like my butt
Laura: Damn it Brendan, why are you so downsy lately???
by ElDub91 May 09, 2011