All 3 Uses

discredit
in
A Connecticut Yankee in King Arthur's Court
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I had a great desire to rack the executioner; not because he was a good, painstaking and paingiving official,—for surely it was not to his discredit that he performed his functions well—but to pay him back for wantonly cuffing and otherwise distressing that young woman.†
Chpt 18

discredit = damage the reputation of
All that is original in us, and therefore fairly creditable or discreditable to us, can be covered up and hidden by the point of a cambric needle, all the rest being atoms contributed by, and inherited from, a procession of ancestors that stretches back a billion years to the Adam-clam or grasshopper or monkey from whom our race has been so tediously and ostentatiously and unprofitably developed.†
Chpt 18

discreditable = tending to damage the reputation of

standard suffix: The suffix "-able" means able to be. This is the same pattern you see in words like breakable, understandable, and comfortable.
But that should be nothing to his discredit; the man that can do this kind of miracle knows enough to keep hotel.†
Chpt 22 *

discredit = damage the reputation of

Definitions:

(1) (discredit)
damage the reputation of -- often causing distrust of or disbelief in
(2) (meaning too rare to warrant focus)