Bosola: i fell into the galleys in your service: where, for two years together, i wore two towels instead of a shirt, with a knot on the shoulder, aft

BOSOLA. I fell into the galleys in your service: where, for two years together, I wore two towels instead of a shirt, with a knot on the shoulder, after the fashion of a Roman mantle. Slighted thus! I will thrive some way. Black-birds fatten best in hard weather; why not I in these dog-days? what do these lines said by Bosola in act 1 scene 1 of The Duchess of Malfi imply ?