Yoink is a multiplayer medal in Halo: Reach, revealed in the 05/28/10 Bungie Weekly update.[1] It is currently unknown what its requirements are.
Its appearance is superficially similar to the Maybe Next Time Buddy achievement in Halo 3.
It may possibly be a reference to a Red vs Blue episode, in which Sarge manages to steal Andy and then Lopez while Caboose is facing the other way, both times saying 'Yoink' or a variation on the term, 'Double yoink'. In this case, the medal may be unlocked by killing an objective carrier and then picking up the objective.
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