Where does “mab” come from?

mab (Welsh) comes from Old Welsh map, from Proto-Brythonic mab, from Proto-Celtic makʷos, from Proto-Indo-European mh₂ḱwos, from Proto-Indo-European meh₂ḱ- — to raise, increase; long.

mab (Welsh): boy, son, infant, child of either sex, minor, youth