メス(mesu, original word 雌) means female(I think it's usually used for animals) in some context can also means feminine
ガキ(gaki, original word 餓鬼) means brats
So the terms メスガキ(mesugaki) basically means female brat
メスヤンデレ(mesu-yandere) according to the definitions would mean female yandere
But...