Used in place of stupid or silly. I believe this is a very loose translation of the dictionary definitions for foolish talk, humorous old man. Anyone a bit silly became a gaffer, and, ergo, some silly thing or act was gaff. Popped up in SE Mass around 1984. Example: Did you see that guy arguing for an hour over the lightsaber fights in _Star Wars_ and how in _Empire_ they cut through the railings, and in _Jedi_ they didn't? What a gaffer! Have you ever see a thing called a Chick Tract? How utterly Gaff!
GAFFER, GAFF
Meaning of GAFFER, GAFF in English
Slang English vocab. Английский сленговый словарь. 2012