A few possibilities:
1. A nickname of
John or possibly
Jacob, through the nn
Jack or
Jake.
2. A nn of
Jacek or
Jacenty, although I believe
Jacek and
Jacenty aren't properly pronounced with a
JAY sound.
3. A nn of
Jason.
3. A form of
Ace formed by simply adding a J.
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Miranda
"Multiple exclamation marks are a sure sign of diseased mind" -- Terry Pratchett