What does "a sock full of pennies" mean?

I tried Googling it, and the only references that came up were to the Seinfeld episode. Furthermore, since I personally have never heard that phrase before, I would assume that it's not an idiom or common slang.

I think the characters are referring to a literal sock full of pennies. After all, getting hit with a sock crammed full of pennies would really hurt. :D

A sock full of pennies (or quarters), when swung by the leg-end would hurt.

http://www.desivideonetwork.com/view/1md0fjd7c/a-sack-of-paisa-with-masala/

A sock full of pennies literally means that one takes a long sock, fills it with pennies, and uses it to hit someone by swinging it at them. Other fillers would include other coins or even batteries. In the associated Seinfeld episode, Joe Mayo was literally attacked with this weapon.

"A sock full of pennies", refers to the act of filling a sock full of pennies, to use as a weapon.

Usually, it was filling the sock full of sand, so that it can be used as a sandbag to slug the back of someone's head. Obviously the "sock full of pennies" was being used for a sinister purpose, as Elaine said that the person was "waiting inside his apartment for him".

The "sock full of pennies" was an improvised sandbag in order to have a weapon to inflict damage.

Filling a sock with hard objects is a common improvised bludgeoning weapon. Just in the last year, see this (using gravel) and this (using rocks). Also I remember watching one prison movie from the 80's where the "hero" beat up some (expected) assailants with a sock filled with freshly bought (still full) pop cans.

It's a reference to the film Death Wish, which is often referenced in the series. In it, the protagonist Paul Kersey(Charles Bronson) asks for $20 in quarters(so 80 quarters) and then he puts it in a sock. When someone tries to mug him, he beats him with the sock.