Interesting. Here's an article that suggests Ephraim wasn't originally a Jewish name at all:
"I would venture to say that it is entirely possible- if not probable- that Menashe and Ephraim were in fact Egyptian names and not Hebrew. Joseph himself was not known to the Egyptians as Joseph but rather by his Egyptian name tzofnat paneach. Firstly, why would Joseph give his sons Hebrew names? It seems unlikely he would burden his children with something that would be nothing less than a handicap for them; Egyptians were not known to be too friendly to foreigners and jealously guarded their own culture. Not to mention the fact that Joseph was the viceroy of Egypt! Why would the viceroy of Egypt give his children Hebrew names? "
"Let us take a look at a similar example of a name whose etymological root is given as Hebrew but is more likely Egyptian, namely Moses. In Exodus 10, he is said to have been named Moses by the daughter of Phaaroh, “ki min hamyaim meshitihu” literally because he was pulled from the water. The obvious problem with this explanation is how could Pharaoh's daughter have known the Hebrew language? and even if she did, why would she give him a Hebrew name? An additional problem lies in the fact that we don’t see the word masha (as in pulling), anywhere else in tanach (other than one obscure passage in Psalms 17: yamsheini mimayim rabim).
Therefore a much more logical explanation would be the one given by Ibn Ezra (and many other commentators) on the verse in Exodus, namely that Moshe was in fact an Egyptian name ."