Fredegar's name doubtless shares its first element with common names like 'Frederick', deriving from frid, meaning 'peace'. The second element is difficult to translate, but perhaps comes from ger, 'spear' (as in the name of his great-grandfather Rudigar). As a whole, 'peace-spear' doesn't make apparent sense, though perhaps it was meant in the sense of, say, a retired spearman.
Whatever its source, it is known that Tolkien didn't invent the name 'Fredegar' out of whole cloth. Instead, it seems to have its source in the name of a real seventh-century Frankish chronicler.