The off-hand rubber chicken is the off-hand variant of the 2005 Easter event reward, the rubber chicken. It is dropped on members worlds by chickens, but only ones that respawned using an animation where they fall from the sky and say "I have returned!". It can also be obtained by players who do not have the original rubber chicken.
Wielding the off-hand rubber chicken does not allow players to whack other players, nor does it allow players to perform the chicken dance emote. If you drop or lose your off-hand rubber chicken, you must kill chickens and obtain it as a drop again.
Although this item is non-members, only members can obtain it as a drop. It is classified as a non-member item because players can un-equip and re-equip it on non-members worlds.
You can only acquire 1 off-hand rubber chicken. If you've already got one, a 'I have returned' chicken can spawn, but will drop nothing on death.
To acquire more than 1 off-hand rubber chicken you will have to drop any previous obtained off-hand rubber chickens before killing an other 'I have returned' chicken, and pick the dropped off-hand rubber chicken up after killing the chicken.