An excessive and/or irrational fear of weapons.
Like most phobias, diagnosis is a matter of degree - being afraid when someone actively threatens you with a weapon is not a pathological response but breaking down into a screaming panic under the same circumstances might be. Being afraid1 of a weapon at rest (for example a gun lying on a table) definitely is.
Likewise the fear of theoretical weapons - an unpopular politician is not phobic in having his car armoured if he has offended enough armed people, nor is someone who is likely to be held up at gunpoint disordered if they decide to wear a ballistic vest. A private citizen who worries constantly about being the victim of a spree killer, or is kept awake at night be visions of being the quarry for his neigbour's duck gun probably is. The mall ninja who tapes ballistic plates to himself as protection against imaginary snipers is delusional rather than phobic.
Note that this phobia doesn't just apply to firearms - although they remain the most popular trigger - knife panics are quite common and even crossbows have been the target of hoplophobia in the past - albiet, like many gun panics, stirred up by politicians2 who preferred not to let the common herd have access to anything that made them harder to oppress3.
Game and Story Use
- Something of a roleplaying challenge for most PCs, but a very interesting trait for a friendly NPC to possess if they need to be handled in a dangerous environment without putting them into phobic panic every time someone draws a weapon.
- In a mainly non-combat campaign it's a good trait for those suffering from various forms of conflict trauma. For real irony it can be attached to the PC with the highest combat skills (to represent a highly shell-shocked veteran) - when he is then needed to protect the rest of the group, he will have to struggle with his phobia just to pick up a weapon.
- A mild form of hoplophobia might be a useful penalty to inflict on a character who takes "pacifist" style disadvantages for extra points and then 'forgets' to play them.
- The political "form" of this is just another unpleasant trait to give to authoritarian or lawful stupid societies. Or, indeed, to a Utopia full of cloud-cuckoolanders (think of Barbarella's response when she's told she will need to carry weapons - and she's meant to be some kind of secret agent - or the horror of the "police" in Demolition Man when faced with an armed opponent intent on an MDK).