I was born in 1963. My mom was always asked if she named me after the movie by Alfred Hitchcock, Marnie. She said no, she was born before the movie came out. She said she had a friend in high school named Marti and liked her name but wanted to soften it up. I've loved having a unique name, though most of my life people, especially teachers, have pronounced it wrong.
Aside from the black mark given this name by being in a Hitchcock thriller, I love this name because it means Rejoice. I wouldn't mind being called rejoice every day of my life. Maybe that's why I've adopted Joy as my new middle name. Any girl would be lucky to have this name. If I give all my characters my favorite names I won't have any left by the time I have children.
No it does not mean rejoice in Hebrew. Sorry (Mitztaer). Marnin is a new way of saying "enjoyable" but a. Barely used, b. Nothing to do with Marni. (Not a typical shortening for such a name).