Acronym typically used in texts or online forums for "You know how I know you're gay?" popularized in the movie: The 40 Year Old Virgin. Typically used when a male acquaintance or buddy does something gay like admit to liking musicals, floral colors, or talk about another man's physique.
Rick: What's YKHIKYG?
Kevin: Because you don't know what YKHIKYG is.