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.
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.
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?