1 They held this land, and with their filthiness
filthiness > obscenity, moral defilement
2 Polluted this same gentle soil long time:
gentle > noble
3 That their own mother loathed their beastliness,
That > [So that] their own mother > [the soil of Albion]
4 And gan abhor her brood's unkindly crime,
gan > began to; did unkindly > unnatural crime > sins (collectively)
5 All were they born of her own native slime;
All were they > [Although they were] slime > slime, soil; flesh (cf. 209.21)