This is a Latin phrase, but certainly not something Caesar or Cicero ever said. It means “God be with you (plural),” and was said a lot as Christianity began to dominate the Latin-speaking world, and continued to be said through the Middle Ages among the clergy, who like all literate people of the day knew Latin.
The phrase breaks down like this. “Dominus” means “God”, and is not capitalized in Latin because capitalization was just not done in Latin, except in modern versions of Latin texts which are adjusted to accommodate the tastes of modern readers.
“Vobiscum” is a bit more complex. It’s a combination of “vobis,” the dative (indirect object) and ablative (instrumental object) form of “vos”, which is the plural form of “you” in Latin, and “cum” which means with. As in many languages, words often combine automatically, so “cum” joins “vobis” to form “vobiscum” whenever they are used together. The same happens with “mecum” and “tecum” (with me and with you (singular) respectively).
A variant does exist: dominus bobiscum. A Google search yields under 200 hits for dominus bobiscum, suggesting it is quite rare. By contrast, dominus vobiscum gets about 125,000 hits.
This alternate is in all probability a simple spelling error. Although “v” and “b” are pronounced virtually identically in Spanish, there is no reason to suspect any spelling difficulties among Spanish speakers.
Instead, on a QUERTY keyboard, the “b” and “v” are adjacent, so the couple hundred hits that “bobiscum” produces are probably just a result of typos. But never underestimate the power of a typo to produce another word.
So a "b" instead of a "v" here for the well-known phrase "dominus vobiscum". Is the "b" spelling is common in the Spanish-speaking world as a variant of the standard "v" spelling? In English, the word "apron" for instance was once "napron". But with people saying "a napron" and not spelling consistently, the "n" shifted off "napron" to the article, giving us the modern "an apron". Something similar could be happening with "boviscum".
For now though, the phrase is “dominus vobiscum” and will likely remain so since most people see it in print rather than hear it spoken.