in mongolian khalkha means shield also it used to mean cheek in ancient mongolian i think the etymology of the name khalkha was extensively discussed in 's if i'm remembering the name of the book correctly khalkha as a geographic entity was the centre of the mongol possessions and therefore the safest place for the mongols to live perhaps those in the southern domains were more exposed and vulnerable to the ming invasions while those in the western domains were vulnerable to the raids from mogulistan and other turkic states