Is not a Portuguese word. Is a joke that actually is a version of another joke. In Brazilian slang there is an expression "é nóis". This expression ("é nóis") is a totally wrong grammar that is part of the expression "É nóis na fita" that means "we are good" or "we can do this". So this year, players started to use the expression "é tóis". None of these expression are portuguese words ... they can be considered Brazilian slangs.

I'm Brazilian and I didn't know the meaning of this expression until today. It's not a Brazilian word, but let me try to explain to you. This #étois thing was created by Neymar (Brazilian football player). When he scores a goal, he goes to the crowd and make a T with his two arms. When he was asked why he does this, he explained that he and his friends when chatting use a slang, which is "É Nóis". It's a expression used for some Brazilians that means something like "we're together". But instead of a N, they used it with a T. That's why he does the T thing with his arms. Why did he replace the N with a T? I don't really know. Here it is the story explaining this whole thing (in Portuguese): http://globoesporte.globo.com/futebol/times/santos/noticia/2012/03/neymar-desfaz-misterio-e-explica-comemoracao-com-t-de-toiss.html