thereby > near by
4 Therewith redoubled was his raging ire,
Therewith > With that; thereupon
5 And said, "Ah wretched son of woeful sire,
sire > father
6 Do you sit wailing by black Stygian lake,
Stygian lake > (The River Styx, the river across which dead souls must pass)
7 Whilst here your shield is hung for victor's hire?
hire > wages
8 And, sluggish german, do your forces slake,