Similar to a lemon party, this sexual act involved older and younger people, however it is an elderly lady with largely saggy boobs that is getting her 'gobs smacked' by a younger male.
Much like a kitten plays with a yarn of wool, batting it around in fascination, the young male smacks the old bat's boobs however he likes
It is also when an old lady wants to get a bit frisky and use her saggy boobs to smack a young male's gob or other parts of his body.
Male: Dude last night an old lady gobsmacked me, it was so good!
Old lady: Oh darling you wouldn't believe how well that 20 year old gobsmacked me last night, it was delightful!