Usted ustath?

Another lightbulb moment: in Arabic, ustath means teacher or instructor and is used an a generic honorific. In Spanish, Usted is the formal address for you, as opposed to the familiar tu. Ustath --> Usted? Seems possible. Especially when you consider that in some regional pronunciations of Castilian, the "d" sounds like a "th."

I need to find a good source or text for the etymology of Spanish words. Is there a Castilian equivalent of the OED? There must be, I'm sure, maybe published by the Real Academia Espanola. *off to check*

ETA: according to everything I find online, usted is a contraction of the phrase "vuestra merced"...so I'm thinking the sound similarity and usage are coincidental.

And the etymological dictionaries of Spanish that I can find online? Range from $51 used to $1,895. The one that I would love to have is $375. Not in my budget right now.

Never mind.