''I am bound to say that in all the accounts which you have been so good as to GIVE OF my own small achievements you habitually underrated your own abilities.''
The two words don't actually form a phrase. They belong to separate clauses, though this is understandably a tricky sentence to parse (took me a minute too). Let me rephrase and simplify it a bit:
In all [your] accounts ... of my achievements, you habitually underrated your own abilities.
Now we can see that "give" and "of" are being used separately with their usual meanings.