A Butter-jug is when you put a stick of butter up the anus of your partner. Then once your partner has entered the position of standing on his/her head, lean him/her slightly forwards. now wait for the butter in their anus to melt and drip down their gooch and onto your tongue.
Note. The residue left behind by the butter would make great anal lube.
(guy no 1) - " Man, i really feel like a good old butter-jug right now.
(guy no 2) - "That's what i was thinking!!"