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2.
Phrase used to describe the incestual fan pairing of Simon and River Tam; both characters from Firely, a short lived sci-fi series, and Serenity, the movie sequel. River is Simon's 17 year old little sister, who became mentally and emotionally unstable after the government experimented on her for three years. Simon, a gifted young doctor, gave up his entire fortune and future to rescue his sister, tearing apart his family as a result. They both became goverment fugitives after River's rescue. There are debates as to whether hints of the pairing ever showed up in the show or not.

As a decision as whether or ot Tamcest exists is up to you.
Possible, completely unbaised, scenes and quotes that fans have interpreted as Tamcest:

In a deleted scene from "Our Misses Reyolds" River asks that Sheperd Book marry her and Simon. When Simon explains that they can't marry, River is still insistant o them "mating" or "doppling" as she spews adjectives for marriage. After a lengthy argument and a swift kick in the shin (by River of course) River stuffs a pillow up the front of her shirt and states that they must marry because she is "in the family way."

In an interview, and I paraphrase, Summer Glau stated that, in the future, she would "like to play another Whedon character that was not in love with her brother."

It is often rumoured, that Whedon stated in an interview that he had originally had a scene where River kissed Simon.

However, all of these can be made invalid if you account for River's mental instability, and that if she does like him, the end scene of Serenity between Simon and Kaylee proves that it was unrequited.
by Poetic Ramblings July 15, 2010
 
1.
Fanfiction that imagines relationships between Simon and River Tam from the science fiction/western, Firefly and it's subsequent movie, Serenity. Both were created by Joss Whedon, creator of Buffy: The Vampire Slayer. There was actually no evidence of Tamcest in the series as Simon and River were brother and sister and that would have been really creepy. There have been few, if any examples of Tamcest and Tamcest stories are not easily accecpted fanfiction.
I read such a good Tamcest last night! River and Simon were getting it on!
by Terri D. November 08, 2006
 
3.
Fanfiction that imagines relationships between Simon and River Tam from the science fiction/western, Firefly and it's subsequent movie, Serenity. Both were created by Joss Whedon, creator of Buffy: The Vampire Slayer. There was actually no evidence of Tamcest in the series as Simon and River were brother and sister and that would have been really creepy. There have been few, if any examples of Tamcest and Tamcest stories are not easily accecpted fanfiction.
Wow, this Tamcest fanfic has a great description of Simon and River getting it on.
by Terri D. November 08, 2006