A true example of a coward, One who often backs out of things and rarely shows they have any balls; you can also refer to someone with no backbone as a pile of goop (if any human didn't really have a backbone, they would simply be a pile of goop)
ME: Hey, can you go upstairs and get my keys?
brother: NO I CANT I'LL BE LATE FOR MY HOT DATE!!!
ME: Aww, you have no fucking backbone
father: Son, are you gunna go on the roller coaster?
son: No! I'm too afraid. I'm gunna go on the kitty roller coaster.
father: Son, you my friend have no backbone
sergeant: WHERES PRIVATE WILSON!
private no.2: Umm, he went home sergeant. He had asthma.
sergeant: Wilson just had no backbone
ME: YOU DIDN'T GO UP TO HER AND GET HER NUMBER? KID SHE WAS DIGGIN' YOU...YOU HAVE NO BACKBONE