what does "Get thee behind me devil and push" mean?
Get thee behind me devil and push, that's what his father used to say every time he unscrewed the lid off the bottle of whiskey. Not that he did it that often, he was a careful drinker. But he took great pleasure in the drinking, he took pleasure in everything he did. The cackle of laughter he would let out of him, Bruno could still hear it. Get thee behind me and push.

Question

Answer

There is a Christian story in the Bible in which Jesus is being tempted by Satan/the devil. He responds by saying, "Get thee behind me Satan!" which is a rebuke or refusal of temptation - a sort of way of saying, "NO!" to the devil's temptations.

MY INTERPRETATION OF THE MEANING HERE:
In this passage, I would say that the father is acknowledging the "evils" of drinking and also acknowledging his "friendship" with alcohol. Instead of saying, "Get thee behind me" to rebuke the devil, he is saying, "Get thee behind me AND PUSH" (Get behind me, my "friend",  and help me get through this).