One-Armed (3.5e Flaw)
From D&D Wiki
You spent so much time working on one arm's strength that your other arm has suffered from lack-of-use.
Prerequisite: Must have functional arms.
Effect: You have one strong arm, which allows you to use two-handed weapons with one hand, but because your other arm is basically limp and has shrunken in size, you cannot wield any weapon with two-hands, and thus do not gain a bonus to strength from doing so. Furthermore, you suffer a -8 penalty on checks related to tasks that would require two hands (climbing, rope use, siege equipment, balance, ride, etc.).
Roleplaying Ideas: A barbarian living in the wild has gotten so used to performing tasks with his right arm that it grew two to three times its original size in muscle mass, allowing him feats of strength, but not without the detriment to his left arm.