Origin of a surname
If we talk about the origin of the surnames, it is an easier topic, or nearly so.
First of all an honest genealogic agency will let you know that, in most cases, you can only formulate hypotheses that can approach with less or more likely the source of the surname.
We must consider that, even if there were some ancient writings, where you can read “the day … this was called … for the reason that …”, this information should always be taken, or nearly so, with reservations.
The ancient writings are the work of humans, then the typical human errors could be present.
Errors like distraction, misinterpretation of the sources, or personal interests and anything else we can imagine belongs to humans.
However, in general, at the end of Middle Ages, XV – XVI centuries (1400-1500), has resumed the habit to give a surname for families.
The surnames were derived from the progenitor craft or trade, or from his nickname.
The surnames could also be modified during the transcriptions in the public registers (the Municipality Register Office document or the Parish register).