The term Gilby originated in South Carolina when a red neck man by the name of Gilby made himself famous by creating the world's most alcoholic moonshine. This moonshine was so alcoholic that it had to be sniffed from the end of a match submerged only a tiny amount into it without breathing in or sniffing any of the large amounts of fumes it would create. Unfortunately, on trying this, Gilby was drunk and he ended up sniffing just one tiny bit of the fumes, resulting in his death.
An alcoholic (or someone who has harmed them self from excessive alcohol consumption) is called a Gilby because of this
Jesus, look a that cunt throwing up everywhere
He's being such a Gilby ...