John and Jonathan, Explained
Not only is one not a shortened form of the other, but the two names are unrelated.
The Christmas season brings to mind, for reasons rather impressionistic, the fact that my first name John is often thought to mean “God’s gift.”
It actually did not begin meaning that, precisely, and the idea that it did is rooted in a common misimpression that, on some level, John is a shortened form of Jonathan. Often, people wanting to address me with mock formality will lengthen my name to Jonathan, as if this were the equivalent of calling a Jim James or a Bob Robert.
In fact, Jonathan is the name that means “God’s gift,” or actually “Jehovah has given,” from an original Hebrew Yonatan, in which the yo is short for Jehovah and natan means “he has given” (Natan alone yielded Nathan, and the t-n consonant root also yielded the Hebrew name Matan).