When the Torah mentions the names of the spies , everyone is referred to as “so-and-so ben (son of) so-and-so.” The only exception is Joshua, who is called Joshua “bin” Nun. Why bin and not ben?
Nachmanides (Rabbi Moshe ben Nachman, 1195–1270) points out this unusual vocalization, and suggests that the two words should be read together as “binnun.” This name, rooted in the Hebrew word binah, means “the understanding one,” and was accorded to Joshua out of respect for his keen intellectual abilities.
Some other explanations that I found:
• The Torah tells us that Joshua was Moses’ student par excellence. “His attendant, Joshua bin Nun, a lad, would not depart from the tent [of study].”
We are taught that one’s students are considered as his children. Some use this idea to explain why Joshua was called bin Nun. He was Nun’s biological child, but he was, to a certain degree, the son of Moses as well. This “dual parentage” is hinted in the unusual way the Torah refers to his relationship with his biological father.
• Chatam Sofer (Rabbi Moses Sofer, 1762–1839) offers another homiletic explanation. According to the Midrash, when G‑d changed Sarai’s name to Sarah, the Hebrew letter yud, which was removed from the name (to be substituted with a hei), felt wronged. Why was it no longer part of the name of this holy woman? G‑d placated the letter by promising to make amends at a future date. This was accomplished when Moses added the letter yud to Joshua’s name (Yehoshua), which was originally Hoshea.
In its original state, when it was the final letter of Sarai’s name, the yud did not have a vowel—as is the case with most letters which come at the end of a Hebrew word or name. Now, in order to be the yud which begins the name Joshua, it would need the sheva vowel, which is comprised of two dots vertically aligned. These two dots were “borrowed” from the segol vowel which normally is beneath the word ben, which is three dots set up as a triangle. This leaves only one dot for the word ben. One dot is the chirik vowel, which changes the pronunciation to “bin.”
Rabbi Baruch S. Davidson