Onion Jar
That guy who knows absolutely everything yet lacks any ability to apply said knowledge. He can tell you everything about the processes gone into making the glass jar, how many onions the jar contains, chemical composition of the pickling vinegar, what type of onions they are .... yet has the complete inability to take the lid off.
"Here Jeff, that new project manager is a right ONION JAR, hadn't got a clue which way to turn the spanner to undo that nut!"