With an obscure meaning of being too happy, overjoyed does come from the verb “overjoy” – to transport with gladness.

• 12.018 Balin was overjoyed to see the hobbit again,

“overjoy, v.” OED Online. Oxford University Press, June 2015. Web. 14 July 2015.

“overjoyed, adj.” OED Online. Oxford University Press, June 2015. Web. 14 July 2015.