The literal translation of the term forbidden
is circumcised.
The word is used widely in the Old Testament to define someone who is outside the holy community. It is also used metaphorically of things that are inappropriate, or that do not work correctly (see, for example, Exodus 6:12, Exodus 6:30; Deuteronomy 10:16). Understood in this way, the word means that the fruit is taboo and not to be touched.
23 “When you come into the land and plant any kind of tree for food, then you shall regard its fruit as forbidden. Three years it shall be forbidden to you; it must not be eaten.