A mildly derisive euphemism related to "pissed off" but with a suggestion of childhood nervous incontenence or bedwetting. Suggests a loss of control from fear or anxiety as much as anger. "Don't get wee-weed up over it" is similar to "don't wet your pants." Origin is the childhood term wee-wee, to urinate.
''There's something about August going into September where everybody in Washington gets all wee-weed up,'' Obama told an audience at the Democratic National Committee headquarters. ''I don't know what it is. But that's what happens.'' -- NY Times, August 21, 2009