In Sir Gawain and the Green Knight, I would like to know what the following passage means."As I heard it in hall, I shall hasten to tell anew. As...

For this excerpt from Sir Gawain and the Green
Knight, this is how I translate
it:

"As I heard it in the hall" means
that the speaker heard something, and "I shall hasten to tell anew"
means that the speaker wants to quickly tell what he has heard (in the
hall).

For the next part, the three definitions below may
help:

readability="7.8214285714286">

href="http://www.merriam-webster.com/dictionary/fealty">fealty:
the fidelity [loyalty] of a vassal or feudal tenant to his
lord

href="http://www.merriam-webster.com/dictionary/derring-do?show=0&t=1298501812">derring-do
- daring action :
daring

href="http://dictionary.reference.com/browse/meetly">meetly
- suitably; fittingly; properly; in a seemly
manner

I take "As
it was fashioned fealty in tale do derring-do" to mean that what he heard was
a daring tale of loyalty and bravery. "And linked in measures meetly letters
tried and true" means that the story's words and descriptions are "meetly
letters," things that are proper and appropriate: words that are "reliable and true,"
having stood up to the test of time.

I would expect that in
this, the storyteller is giving the qualifications of this story: presenting the story's
credentials, in a way—saying that it is a true story, and the listeners can be sure that
what they hear is the
truth.

Additional
sources:

http://www.merriam-webster.com/dictionary/fealty

http://www.merriam-webster.com/dictionary/derring-do?show=0&t=1298501812

http://dictionary.reference.com/browse/meetly