Literal translation
do good to Bertrand, he repays you by letting you down
Meaning
do good to Bertrand, he repays it with ill treatment
Explanation
This saying denounces the ingratitude of someone who is helped, using the proper name Bertrand as a conventional, anonymous stand in, much like an unspecified Tom or Dick. The words to do, well, to, you, it, and to give back are all well attested, while the name Bertrand, like any common proper name, naturally has no place in an ordinary vocabulary lexicon. The last verb, rendered here in the sense of letting down, failing, or backing off, carries a considerably coarser meaning in its primary sense, a familiar term for excrement; the saying, however, keeps only the figurative, softened sense of failing or disappointing, the sole way to phrase the image decently. The saying illustrates a universal theme of folk wisdom, that of a good deed unrecognised, even turned against its author by the very person who benefited from it. It is used to comment, with a resigned bitterness, on a disappointment following a service rendered in good faith.