It should be "could have said" Using "of" is incorrect, but I have seen it used on sites such as Facebook, by people who can't write their own language correctly! I think it is because we say "could've said" quite quickly, and the "have" gets lost, and then somehow gets written as "of"! But it is wrong! :-)

It's a terrible and laughable mistake. Don't ever write it. "Well you could of said that before!" and "Well you could've said that before!" sound exactly the same, but only "could've" is correct. "Could of" doesn't even begin to be logical. This is one of the many ways how you may see who failed at school. :)

"Could of" is a non-standard form of "Could have." Today, a lot of people write "could of" because this sounds like the contraction of "could have," which is "could've." So, in terms of standard/proscribed grammar, the sentence should be: You could have said that before. But in popular speech, especially on the internet, you will see "could of" a lot. Technically, as far as proscribed grammar is concerned, "could of" is incorrect. The same applies to should of (should have) would of (would have) might of (might have) Nevertheless: Some famous writers, such as John Dos Passos, have used the phrase: "Could of" in literature to mean "could have" in the sense of mimicking the pronunciation of the phrase and/or rendering the speech of the common ("less educated") person. On a grammar test or Test of English, use "could have" and not "could of." But, because language is always changing in accordance with how people actually use the language, it may well be the case that in 100 or 200 years, "could of" will be accepted as a standard form.

This is used by Americans a lot. I have read it in fiction from the 1950's, so it has been in use for at least that long. It is also used by the less educated in the UK. 'Could have' not 'could of'

Basically, what it means is: "You should have said that before this time." If you had told me that before, I would not have wasted time. The person knew something that would have made things easier for the person speaking, but didn't say anything.