derect sunlight
A boner that you get while sunbathing nude, usually in response to seeing naked ladies in your same vicinity.
People consider it perfectly acceptable/understandable to "publicly" have a hard-on in areas of derect sunlight (i.e., a "clothing optional" beach), but it is still frowned upon to ask for sex from fellow tanners of the female persuasion, even though they, too, are totally "showing off their merchandise" for all to see, and thus it would be only natural for red-blooded studs in their vicinity to "want some" from them.