1. From the place name "Carbajal": The surname might be a habitational name, meaning it originates from a place called "Carbajal". This is the most likely scenario, as there are several towns and villages in Spain named Carbajal, such as Carbajal de la Legua in Madrid and Carbajal de la Fuente in León.
2. From the word "carbajal": This word is a regional term for a type of "oak" tree in the north of Spain. It's less likely the surname derives directly from the word, but it could be connected to the tree in some way, possibly related to a place where oak trees were abundant.
Ultimately, the exact meaning of the surname "Carbajal" is unknown, and it's likely the meaning varied depending on the family's specific origin. However, it's safe to say that the surname is of Spanish origin and likely connected to a place or a type of tree.