Spanking The Monkey
This easter egg is found in Season 2, Episode 23b.
In Season 2, Episode 23b of Rockos Modern Life, Mr. Big Head asks Rocko if he and his friends (Heffer and Filbert) would like to play a game, referring to bowling. However, when he asks Rocko, Rocko responds stating, “We’re already playing a game”.
The game in question that Rocko, Heffer and Filbert are playing turns out to be “Spank the Monkey”. In the pictures below you can see the game involves spanking a bare butt monkey so it turns around on the perch its built on.
However harmless the game may seem, “spanking the monkey” is actually a well known slang term for masturbation.