It is known that ancient Greeks and Romans, on drinking occasions, poured out wine libations on behalf of the dead. I have seen this practice still in Romania quite commonly. When wine is set forth, someone will lead the group in pouring out a bit onto a plate or the ground (if outside), with the words pentru morți (for the dead). So we can be safe in assuming that when ancient Romans performed this ritual they poured out wine and spoke the words "pro mortuis."
[Worth memorizing here is the Latin proverb De Mortuis nil sed bonum, 'Concerning the Dead, [speak] nothing but good'.]
But, in addition to the dead, the ancient Romans must also have been making toasts for the health of themselves and their guests. And the common words in the Romance languages for this toast points us to the Latin original.
The Romans undoubtedly made a toast for the health of each other using one or both of the words sanitas and salus.
[Worth memorizing here, salus populi suprema est lex, 'the safety of the people is the highest law']
The Latin toast word prosit (may it be good for someone) has come into German as both prosit and the shortened form prost.
In my novel In Saecula Saeculorum, four high school students are sent back in time to ancient Rome on a mission to save the world. But they are not prepared for the fact that this will mean they are drinking wine at nearly every meal. And one of the students, Carmen, quickly develops a problem with alcohol. And this is a bit of a crisis, because she is also the best fighter in the group. The students are surrounded by dangers that Carmen is best suited to face, all while she sinks ever deeper into her addiction. To find out how it all turns out, click on the cover to explore the book further.