Urban Dictionary defines it as follows:
1. A person at a loss for words, staring blankly with mouth wide open
i.e. Claire stood slackjawed as she watched T2 sky a seven-foot Amazonian.
2. A game in which two people try to keep their mouths slack for as long as possible while dancing redonkulously. The person who smiles or closes their mouth first loses. Like a staring contest, only with a slack jaw
i.e. With each woman having two slackjaw titles to her name, Rachel vs. Allison was sure to be an epic, clash-of-the-titans showdown.
3. A women’s club team established in 2007 and comprised of the best current college players and recent graduates from the Bay Area Section, one of the most competitive and highly regarded sections in the country
i.e. Psyched for the 2010 season with a sponsorship from Zone Clothing, Slackjaw continues to develop hard-working, fun-loving, and dedicated ultimate players under the direction of coach Joy “Chewie” Chen and with the guidance of captains Rachel Dyke, Tien-Tien “T2” Chen, and Caitlyn “Ruggs” Rugg.