A form of Sexual Asphyxiation you go down on your partner (Head) while they stand in the shower and waterruns along his or her penis and water (Boarding) you while you pleasure him or her.
"You pair are always at it! I didn't mean to walk in on you head boarding her last night"
"I heard you and your boyfriend in the shower last night."...." Yeah, I totally head boarded him, but he loved it".
the loss of intelligence or capcity for logical thought resulting from prolonged bashing of the head against a headboard during vigorous sexual intercourse; someone's brains have literally been fucked out
Britany Spears' recent(2007) downward spiral is directly attributed to headboard syndrome.