Therewith > With that; thereupon he > [Braggadocchio]
2 And threatened death with dreadful countenance, 3 As if their lives had in his hand been gaged;
gaged > pledged; left in pledge
4 And with stiff force shaking his mortal lance
stiff > violent mortal > deadly
5 (To let him weet his doughty valiance),
weet > know, perceive valiance > valour, bravery
6 Thus said: "Old man, great sure shall be your meed,
meed > reward
7 If where those knights (for fear of due vengeance) 8 Do lurk, you certainly to me aread,