Polished Hand (3.5e Flaw)
From D&D Wiki
Two hands are better than one, so why didn't you train both of them?
Prerequisite: Character must have more than one hand.
Effect: Characters with this flaw take a -2 attack penalty when wielding a weapon in their off hand(s), and when wielding a weapon with more than one hand. They also take a -1 AC penalty when wielding a shield in their off hand(s).
Roleplaying Ideas: You were taught to play the piano from a young age, but you always half-assed it.