A woman of questionable moral and ethical standards who may fit one or more of the following: slutty, trashy, drunk, skanky attire, ugly, etc. Essentially a scally can be considered someone you may sleep with on a one night stand, but would never take to meet your parents. Alternatively, a scally still includes those wenches you wouldn't sleep with at all for any myriad of reasons including those listed above.
Term originated in Loudonville, OH, in 2011.
"Let's go down to Beano's and pick up some scallies."
"You see that scally Corey was with? She as a ho, fo sho."