Hi there. These are just words turned around backwards to make them more interesting. チャンネー is backwards for 姉ちゃん, and シースー is backwards for 寿司 I'm not sure where this phrase came about, but it seems to be common when going out to sushi somewhere in Tokyo. I did a few internet searches and this was all I could come up with. For example, you might say ザギンでチャンネーとシースー 銀座で姉ちゃんと寿司 Sushi with my sister in Ginza. An internet search also brought up an application on Mixi called チャンネーとシースー, which could be where this phrase comes from, but I'm not sure what the application is for. Hope I could help.

おもしろい日本語知っていますね(^^) Funny Japanese! How did you know that! hahaha!!!!!!!!!!!!! The words was used about 20 years ago. Namely very very old slangs. 「チャンネー」means showties.「シースー」means Japanese Sushi. You will be able to understand, if you reverse the part of words. 「チャンネー」→「ネーチャン」 「シースー」→「スーシー（スシ）」 Are you alright? hahaha!!!!!!!!!!

「チャンネル」って言う言葉じゃないか？= channel 频道 I wonder if you could provide the context?