A leffler is someone who is a cocky bastard but usually has good reason to be. Supposedly, the term originated in Georgia around the late 1980's. A leffler is a shmo who can usually talk his way out of anything or into everything.
Girl "Stuart why are you so cocky"
Stu "I am not cocky I am just confident"
Girl slides down Stuart’s pants
Girl "Oh that’s why"
Stu "Care for another rusty hook?"
Girl "Ugh, you such a leffler..but OK"
n. Mythological beast, half horse, half cheetah.
That Leffler can't hit a tennis ball for his life!