A person of only some or no importance to a plot or situation, it's origins sprout from the Naruto Abridged Series, in which a little boy is referred to as Gilligan, and claims that is not his name and is ignored.
Person A: Come along Gilligan.
'Gilligan': That's not my name!
A: It is now.
'Gilligan': But I already have a name!
A: Come along Gilligan.