As a current Harley rider - he could've also meant "slow down" in a tongue-in-cheek sorta way! I am a low waver also but I'm so short Everything about me is low to the ground - even my Harley! Don't feel uncool - you smiled and waved back - thats all that mattered! Most Harley riders use only 1 insulting gesture - everything else is fun/happy! As for being fun and friendly - I've never met an unfriendly Harley rider and that includes the Hells Angels which rode in every charity ride my old AZ chapter put on.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"