Attacking with a non-proficient weapon

I’m sure I’m missing something here, but what happens when (for example) a cleric gets disarmed and someone tosses them a sword to defend with? My first inclination is that anyone wielding a weapon they’re not proficient in fights as a 0-level human, but is there a rule I’m missing that covers such a situation?

ACKS Core, page 39.

Regardless of class limitations, characters of any class may purchase, carry, and use any weapons or armor desired. However, characters who equip themselves with weapons and armor unusable by their class will fight as 0th level characters while so equipped. They also will receive no bonus on their attack throws or armor class from ability scores or equipment, may not use any of their class powers, and do not gain XP.

Thank you - I looked in the weapons section, I looked in the proficiencies section, I looked in the combat section, and I even pulled down the Player’s Companion and looked through the class creation section, but I didn’t think to look in Purchasing Equipment.

ACKS, like the Talmud, requires a lifetime of study.

So, it is the megillah of the OSR?