Trixie McGee was a freckled-face girl that worked at a fish market, who was known to both Morgana and Grandpa Lou in their youth. Trixie apparently had asthma and one day had a terrible asthma attack. Lou, being the only one in sight, gave her mouth-to-mouth resuscitation. Morgana saw and mistook the action for Lou being unfaithful. This caused her to not only dump him but leave town forever, leaving Lou confused and heartbroken.
Note: Contrary to popular belief, Trixie McGee is not the grandmother of Tommy and Dil. A photo was shown of Lou Pickles' wife, Trixie; she had short, brown hair, and looked very different from Trixie McGee. In addition, on the Valentine episode of Rugrats, Lou admitted he met his wife on a cruise. Trixie McGee was from his home town, and was not the Trixie that Lou married.