From Proto-Brythonic *merx, from Proto-Celtic *merkā (compare Cornish myrgh, mergh, Breton merc’h), from Proto-Indo-European *méryos (“boy, girl”) [compare Scottish Gaelic smarach (“lad”), Latin marītus (“husband”, Spanish marido), Ancient Greek μεῖραξ (meîrax, “boy, girl”), Old Armenian մարի (mari, "female bird, hen") and Sanskrit मर्य (marya, “suitor, young man”)]. Related to morwyn ("maid; barmaid").
The Brythonic languages is the language family that Welsh, Cornish, and Breton belong to. Proto-Brythonic is the ancestral language that those three (among others) evolved from - the "last common ancestor", more or less, before it split up into separate languages.
A very historically important descendant of Proto-Brythonic was Common Brittonic, the language that was spoken in most of Great Britain from the 6th century BC to the 6th century AD, when it was gradually replaced by Old English, a Germanic language brought by the Anglo-Saxon invaders. Both Celtic (of which Brythonic is a primary branch) and Germanic languages belong to the pretty extense Indo-European family, which extends from Spanish and Portuguese to Persian and Hindi, and is the world's most spoken language family.
Old English evolved ultimately into Modern English, and Common Brittonic became Welsh, Cornish and Breton.