Almost certainly originates from a TV coffee advert of the 70s/80s starring Gareth Hunt. He holds several coffee beans in a half-clenched fist then shakes them in a way reminiscent of a hand job. The relationship with Gareth Hunt is further strengthened since he was a considered to be a bit of a wanker.
She gave me a Nescafe Handshake
by pilonor January 17, 2009