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'.
The Yoink medal in Reach is awarded when a player steals another teammate's kill when they are attempting the animated assassination.
http://xbox360.ign.com/dor/objects/14276699/halo-project/videos/halor_gmp_multi_0807.html?show=hi