“magnanimous” is a secondary meaning. It MAY be derived by moving from the general idea of a person with a heart as big and strong as a lion. It you then think of “heart(ed)” in the EMOTIONAL sense, esp of loving and caring, you get to the idea of “lion-hearted” as having a heart that LOVES strongly.

But here is, I think, the more likely explanation. Note that “magnanimous” appears in definitions of the word capitalized (Lionhearted), and that in most if not all listings (see the various links from onelook below) it is followed by “Sir Walter Scott”.

http://www.onelook.com/?w=lion-hearted&ls=a

I must assume, then, that these listings (or perhaps all copies of ONE old Webster’s listing) are a specific reference to Scott’s characterization of King Richard I of England — “Richard the Lion-hearted”– in his novel *Ivanhoe*. He portrays him as not only a valiant warrior but also a just and generous ruler, who drafts Robin Hood and his men to assist in rescuing his own people from the deprivations of his brother, King John. (This, by the way, is the source of our modern characterization of Robin Hood.)

http://www.novelguide.com/Ivanhoe/characterprofiles.html (see “Richard the Lion-hearted”)

http://en.wikipedia.org/wiki/Ivanhoe