The rule of thumb:
If a single classed thief cannot use a weapon due to class restrictions, then that weapon cannot be used to backstab with.
Now there are some exceptions to this rule. To be a little bit more accurate (and more complete), weapons you can't backstab with are:
- Axes
- Bastard Swords
- Flails
- Morning Stars
- Halberds
- Maces
- Spears
- Rods
- Two Handed Swords
- War Hammers
Thus, you can backstab with:
- Daggers
- Katanas
- Long Swords
- Scimitars
- Short Swords
- Clubs
- Staves
- Wakizachis
- Ninja-Tos
As for the exceptions:
- You cannot backstab with the Aegis Fang from Wulfgar (AEGIS)
- You cannot backstab with the Drow Scimitar +3 (DWSW1H01)
- You cannot backstab with Haer'Dalis' personal weapons (Chaos & Entropy) (NPSW06 & NPSW05)
- You cannot backstab with the Staff of Arundel (Druids only) (WASTAFF)
- You cannot backstab with the Staff of Curing (STAF10)
- You cannot backstab with the Staff of Power (Wizards only) (STAF12)
- You cannot backstab with Cernd's personal staff, the Staff of the High Forest (NPSTAF)
- You cannot backstab with the Staff of the Magi (Wizards only) (STAF11)
- You cannot backstab with the Staff of the Woodlands (Druids only) (STAF14)
- You cannot backstab with the Staff of Thunder and Lightning (Clerics & Druids only) (STAF13)
- You cannot backstab with the Scarlet Ninja-To +3 (Monks only) (WANINJA)
- You can backstab with Mazzy's Short Sword (Sword of Arvoreen) (NPSW01)
- You can backstab with the Rod of Terror (RODS05)
