When a male unzips his fly and 'whips out' his penis, thus demonstrating how big he is. This can also refer to any situation where a male takes his dick out.
This expression refers to the legendary press conference where, when asked why the U.S. continued to fight the Vietnam war, President Lyndon Johnson "unzipped his fly, drew out his
substantial organ, and declared, 'this is why!'"
For the
metaphorical meaning, see
dick waggling contest