Weapon Specialist (3.5e Trait)
From D&D Wiki
 Weapon Specialist
You've used only one weapon so far.
Benefit: If you have no proficiency with an entire group of weapons(simple, martial, exotic) you lose all of your weapon proficiencies, and gain Weapon proficiency, Improved Critical, Weapon focus, Power Critical, and Weapon Specilization with one simple weapon you choose. If you have proficiencies with all of the simple weapons, you can choose one martial weapon instead. Similarly, if you have proficiencies with all of the martial weapons, you can choose one exotic weapon. If you are proficient with at least one exotic weapon you can choose to loose all of your current exotic proficiencies, and choose one exotic weapon for this trait. The effect of keen is able to stack with the improved critical effect granted by this trait.
Drawback: You lose weapon proficiency with every other weapon.
Special: You can't get proficient with any other weapon in any way. If you want to get proficient with a weapon other then your chosen one, you have to sacrifice a feat at the first available level up, that gives you a new feat.
Roleplaying Ideas: This doesn't usually have any major effects on the roleplay, but a character should give a reason in his background for being proficient with only one weapon. Perhaps your character simply prefers a distinct style of combat they feel is more flavourful. Perhaps as a child they were stranded with only one weapon to defend themselves against wild animals. Perhaps the weapon type was a family heirloom, tradition of weapon type, etc.