The act of slapping something with a semi-erect penis. It's derived from the Dutch verb "swaffelen".
In 2008 it won the title "Dutch
word of the year" in a yearly held competion where the Dutch speaking public can vote online which new word, that originated in that year, they liked the most.
It had gotten a lot of media attention because a Dutch student "swaffled" the Taj Mahal on a school trip; and got expelled from his school for that action.
Recently Brittish carshow host
Jeremy Clarkson mentioned it in a talkshow (BBC Qi), where he gave the definition: "To bang your penis against the Taj Mahal".
He also mentioned the word on the fifth episode of the new carshow "
The Grand Tour" which was hosted from a tent in Rotterdam (the Netherlands).