When a girl sneaks up behind you, reaches around and starts giving you a wristy!

The definition comes from the Aussie TV show - The Surprise Chef - hosted by Aristof (A wrist off)

A Bloomer is someone with a bright outlook on life who is optimistic in nature. Bloomer's frolic in every opportunity to be happy and try to make the most out of life. The opposite of a Bloomer is a Doomer. The term Bloomer originated specifically as a counter to the Doomer on sites especially like 4Chan.