The term "Splunge" has been invented by Monthy Python and has been used in their "Twentieth-Century Vole" sketch. It's in the 6th episode of the first series of Monty Python's Flying Circus, first aired on November, 23th 1969.
In this sketch the famous film producer Larry Saltzberg (Graham Chapman) presents an idea for a next movie to his writers. When the first writer is asked for his opinion about the actually quite trivial plot, he pays absolute homage to him, declaring it as the best movie ever. Saltzberg fires him, as he can't stand a yes-man. Of course, his idea could also be lousy.
So the next writer admits, that the idea is lousy, for which he gets fired as well, because this idea is not lousy, according to Larry Saltzberg.
The next writter, unsure how to comment the idea, gets fired for being indecisive.
The fourth writer, now in total dispair, finally exclaims "Splunge". Asked for the meaning of "Splunge" he explains that it means "It's-a-great-idea-but-possibly-not-and-I'm-not-being-indecisive", which is accepted by Mr. Saltzberg. The remaining writers all agree, that his idea is "Splunge".