The phrase “the fruit of my loins” (or variations of this phrase) appears 20 times in the Book of Mormon, with 19 of them being found in 2 Nephi 3. For example Lehi speaks to Joseph in 2 Nephi 3:3-4:
And now, Joseph, my last-born, whom I have brought out of the wilderness of mine afflictions, may the Lord bless thee forever, for thy seed shall not utterly be destroyed.
For behold, thou art the fruit of my loins; and I am a descendant of Joseph who was carried captive into Egypt. And great were the covenants of the Lord which he made unto Joseph.
Matthew L. Bowen and Loren Blake Spendlove ask:
So, why would this single chapter in the Book of Mormon use this obscure FL (fruit of my loins) collocation with such frequency when it is rarely found in other books of scripture, including in the other books and chapters of the Book of Mormon? The answer to that question largely resides within the stories of Jacob’s adoption of Ephraim and Manasseh as “Israel” and of Jacob blessing his sons in Genesis 49.– Matthew L. Bowen and Loren Blake Spendlove – “Thou Art the Fruit of My Loins”: The Interrelated Symbolism and Meanings of the Names Joseph and Ephraim in Ancient Scripture
Lehi told his son Joseph, “Thou art the fruit of my loins, and I am a descendant of Joseph” (2 Nephi 3:4). The first line, “Thou art the fruit of my loins” parallels the second, “I am a descendant of Joseph.” In essence, Lehi was saying that his own son Joseph, the “fruitful son,” was the fruit of his loins in the same sense that he was the fruit of his father Joseph, the “fruitful son” of Jacob. In a poetic way, Lehi’s son Joseph is both “fruit” (as the son of Lehi) and “fruitful” (named after their common ancestor Joseph, whose name is etiologized with the harvest verb ʾāsap (“gather [in]”; “bring in”; “take away”) and its actual etymological source, the verb yāsap in the sense of “adding” a son or “branch” (bēn) — i.e., “fruit” or posterity. We propose that the abundant use of the FL collocation in JST Genesis 48 and 50 and in the Book of Mormon is best explained as an onomastic wordplay on an allusion to the interrelated meanings of the names Joseph (“may he add”) and Ephraim (“doubly fruitful”).– Matthew L. Bowen and Loren Blake Spendlove – “Thou Art the Fruit of My Loins”: The Interrelated Symbolism and Meanings of the Names Joseph and Ephraim in Ancient Scripture
How would Joseph Smith have known how to create this wordplay?