from the japanese sound effect for "meow" ("nya" in japanese) and the suffix "ko" used exclusively in japanese girls names(i.e.: Ayako, Sachiko): A Catgirl
Nyanko was a catgirl...
The way someone tries to motion to have sex with them, but in a less explicit way instead of hey lets fuck! :D
hey wanna have some nyanko with me? itll be fun.