What does "a sock full of pennies" mean?

I watched Seinfeld S09E12 The Reverse Pipehole, there are lines like this:

KRAMER: Newman and I are reversing the peepholes on our door. So you can see in.

ELAINE: Why?

NEWMAN: To prevent an ambush.

KRAMER: Yeah, so now I can peek to see if anyone is waiting to jack me with a sock full of pennies.

and this:

JERRY: So, Silvio ambushed Joe Mayo?

ELAINE: Yeah, he was waitin' inside his apartment for him with a sock full of pennies.

JERRY: He should have had a reverse peephole.

What does "a sock full of pennies" means? Is it an idiom or slang? Does it really mean a sock full of pennies?

"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.

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.