Ephraim (אֶפְרַיִם) was the younger brother of Manasseh, and they were the sons of Joseph and Asenath, who was given to Joseph as a gift from Pharoah. In Genesis 48:13 et seq., when Jacob finally gets to see Joseph (and I use the term broadly, as Jacob’s eyes were dim by that point - something which apparently runs in the family), he also gets to see the grandsons! And we see history repeating itself with the second son getting the bigger blessing.
Joseph held out his sons to Jacob, and Jacob crossed his hands to bless Ephraim with his right. He then blessed them both. And when Joseph tried to correct his father’s hand placements, Jacob told him that Ephraim’s lot would be greater than Manasseh’s, though both would prosper and multiply. (“Ephraim” comes from the root פרה, “fruitful.”) Apparently wanting to emphasize who would be the future leader of production, Jacob also put Ephraim’s name first in the blessing that we have repeated many, many times since, saying that in the future people would say, “May God make you like Ephraim and Manasseh.”
Shortly thereafter, Jacob called together all his sons and laid out his impressions of each and what was to befall them all. Joseph received an extra portion of land which Jacob had won from the Amorites, and the tribe of Ephraim would be landed in what is now central Israel, between the west bank of the Jordan River and the coast of the Mediterranean.
There are times when Ephraim is referred to as a son of Jacob. This is because in Genesis 48:5 Jacob said, “Now, your two sons, who were born to you in the land of Egypt before I came to you in Egypt, shall be mine; Ephraim and Manasseh shall be mine no less than Reuben and Simeon.” And in Genesis 50:23, we read, “Joseph lived to see children of the third generation of Ephraim…”
Now, about the fused bones. There is a story in 1 Chronicles 7:20-22 that the descendants of Ephraim were killed by the men of Gath (natives of the land) for planned cattle thievery, and Ephraim mourned many days and his brothers comforted him. And the story goes on that their bones (the remains of some 300,000 individuals) were left by the roads, as a warning, and that is one reason that the Jews trekked the long way around after leaving Egypt. Some rabbis also have posited that these were the very bones referred to when Ezekiel in a vision comes upon very dry bones and God asks Ezekiel whether they may live again, Ezekiel passes the buck back to the Almighty as the only one who may know.
But the bones mentioned in the inquiry to your correspondent come from the study section of the Siddur Sim Shalom (slim weekday version) read in morning minyan:
חֲב֧וּר עֲצַבִּ֛ים אֶפְרָ֖יִם הַֽנַּֽח־לֽוֹ׃
“As it is written, ‘Ephraim is fused bones. Let him be’,” which is Hosea 4:17, not from Ezekiel. Interestingly, a different translation says “Ephraim is addicted to images - Let him be.” Non-Jewish sources tend to say “is joined to idols.” And when the old Hebrew is plugged into the new Google Translate, one gets back (but one should outright reject) “a bunch of Ephraim’s nerves.” The word “עצמות” may be translated as “bones.” So we defer to the linguists for the literal translation.
Some have said (Genesis Rabbah 38:6) that the whole notion refers to whether there is peace reigning in the land. If there is peace, God would not mind so much whether the stiff-necked or fused or addicted ones worship idols, for they are redeemed and protected by having established peace. There are other discussions in which the rabbis have said that having the ultimate goal of peace may tacitly permit seeming transgressions, as God values peace above all.
So Ephraim and his descendants seem to be stuck with bones as a heritage, along with a quest for peace.