Even though I'm fairly familiar with British slang, I had to look up ninnycock.
Hehe.
It actually means a young lobster. The implied insult being someone who has no brain and is so inexperienced/naive that they can't even find their way around the ocean bottom.
Incidentally, "gouty-legged ninnycock" is listed in the top 50 insults from the late 1500's. It's making a comeback, too :)