A man's act of hiding his erection by flipping it up and tucking it into the waist of his pants. You flip tuck to conceal your boner, just like people conceal their flaws with a Nip Tuck.
Jim: "Did you see that hot ass girl on Nip Tuck last night?"
John: "YES! Holy shit, I had a boner all day. I had to Flip Tuck every time I left a class!"