Well, here's how it works to the best of my knowledge:
[ITEM_WEAPON:ITEM_WEAPON_SWORD_SHORT]
[NAME:short sword:short swords]
This stuff is self explanatory.
[SIZE:300]
You've gotten this far, so I assume you're familiar with dwarven size units. Just the same as with creatures, just this is for killing stuff.
[SKILL:SWORD]
This determines what skill the weapon uses/trains. There's SWORD, AXE, HAMMER, WHIP, SPEAR, MACE, PIKE and MINING. MINING allows it to be used like a pick.
[TWO_HANDED:37500]
This is the minimum size a creature has to be to wield it one-handed. Any less than this number and it becomes two-handed.
[MINIMUM_SIZE:32500]
This is the smallest a creature can be and still actually wield it.
[CAN_STONE]
This means it can be made of sharpened stone.
[MATERIAL_SIZE:3]
How much material it takes to make it. To my knowledge, 3= one bar of metal.
[ATTACK:EDGE:20000:4000:slash:slashes:NO_SUB:1250]
[ATTACK:EDGE:50:2000:stab:stabs:NO_SUB:1000]
[ATTACK:BLUNT:20000:4000:slap:slaps:flat:1250]
[ATTACK:BLUNT:1000:1000:strike:strikes:pommel:1000]
This is covered in the raws, but I'll explain anyway. EDGE and BLUNT are simple. EDGE cuts, BLUNT whacks. The contact area defines how much of the weapon makes contact with the target. Penetration size, I believe, defines how big a hole it makes. Verbs are self-explanatory. The noun is for if you want the combat message to have a "with x", wherein x is the noun (ie. Strikes with the blade). Velocity multiplier is how much force that attack gets.
In your entity, just take the weapon's identifier and bung it in there where the default was. Hope that helps.