When someone is caught committing a crime (mainly one sexual in nature, but can refer to all crimes), with overwhelming evidence to prove they did it; in their defense, the culprit denies that they were the one caught doing the crime.
Derived from the Shaggy song "It Wasn't Me", which describes a man being caught having sex with another woman by his wife, and subsequently denying that it was him she actually saw.
When caught on video having sex with a 14 year old girl, R. Kelly employed the Shaggy Defense; he just denied that he was the one in the video.
An old woman typically between the ages of 70-90 that is commonly seen on news channels for accidentally crashing their cars into shopfronts. A standard Sheggy is an old women with grey/white curly hair and a pre-2000s car generally ranging from the colours white, blue, gold tint and silver.