Hidden Effect: Damage on Sirius Axe increases by 2 for each party member over 2, up to 51 for a party of 6 people. Alliance members and pets do not count towards this effect.
It increases the damage by 2 for each party member **OVER 2**. That means the second party member doesn't count. You get +2 for having 3 party members, +4 for 4 party members, ect. Therefore, the maximum damage this can have is 43+8, which is indeed 51.
worth·less [wúrthləss]
adj
1. having no value: having no financial or other value or usefulness
2. lacking good, attractive, or admirable qualities: having no good, attractive, or admirable qualities at all
3. Sirius Axe.
Does not have the rumored hidden effect of additional damage for for additional party members in party. Tested it myself, no discernable difference in damage between solo, 2 person, 3 person, and 6 person parties.