If it is a chick? Its just a name. (see first example) BUT...
If it's a guy? He is F'n hardcore. A "Kelly" will drink your drink, smoke your smoke, eat your philly cheese steak and say, "you got any ranch?" with a smile.
If you have a problem with a "Kelly", you won't realize it till he's gone and you were owned. (see second example)
1. Dude, look at all the Kellys here! I will get some leg tonight fo' sho'!
2. Last night, this Kelly was talkin to my girl. Woke up the next day he's driving my mustang and my bookie sayz I owe him $4600.