I have looked into this name a little bit online (as a first name), and at least some usages of the first name seem to be direct simplifications from surname Meredith (and possibly Merriday). I'm not sure about the origin of Merriday, but docs on Meredith's meaning are easy to come by.
Meridy's been used as a first name for both males and females, historically more often for males.
Please post if you find out any more about the surname. I haven't been able to confirm the Meredith theory for sure.