But it's very different to mos rhyming slang. For a start, it simply doesn't rhyme. Also, the usual form of slang has two elements and it is the NON-rhyming element which is used as the slang word. So we have "apples" as slang for "stairs", because of "apples and pears"; and "whistle" for "suit", because of "whistle and flute". So, though it is generally agreed that "say so" is the basis for "I should cocoa", personally I would not call it rhyming slang.