The term "Susan Glenn" is the result of a marketing campaign executed by Axe, the makers of Axe Body Spray. Starting in May 2012, "Susan Glenn" began appearing on various websites - including this one. July 2012 saw the release of video advertisements and the launch of fearnosusanglenn.com, which definitively identified Axe as the creator.
(Interestingly, the definition below from the submitter in San Dimas, CA anticipates the marketing-campaign origin.)
The term hasn't been deleted from this website because Axe's efforts have been successful: this term is now in the vernacular. See the list of tweets containing this term (below) for an illustration.
Because of the interesting origins of this term, the definitions below haven't been edited, and duplicate definitions haven't been combined.
Not just a girl, but the girl - A female who possesses a combination of physical and behavior characteristics that make her infinitely desirable to a man or boy. Susan Glenns are held up as nearly untouchable, and their very being creates vast emotions in the men who covet them.
Man, I remember my Susan Glenn - she wasn't just pretty, something about her really blew me away. I couldn't string three words together coherently around her, but if I knew what I know now?
Who was my Susan Glenn? Heather. She was totally my Susan Glenn.
Definitions include: a humorous response to an attempted fist bump whereby the attempted fist bump receiver does not reciprocate, but rather moves their hand directly underneath the fist, palm down, and raises the first two fingers.