Byron’s use of dashes and sentence fragments in this passage helps to convey the agitated state of mind of Giaffir, who has just quarrelled with his (supposed) son Selim, but then “quailed and shrunk askance” from the look of rebellion that he has provoked in the young man.
In a couple of places in this passage, Giaffir starts to form a thought, but then breaks off without completing it. First we have, “I never loved him from his birth, and–but his arm is little worth”, where it seems that Giaffir meant to add something after “and”, perhaps explaining why he never loved Selim, but instead he broke off to criticize Selim’s skill at hunting instead.
Then, in the queried line, Giaffir starts to form the thought, “he [Selim] hath not heard—” but then cuts himself off by saying to himself, “no more”. The implication is that whatever it is that Selim has not heard is so unsettling or uncomfortable, that Giaffir prefers not to complete the sentence, even in thought. This creates suspense for the reader, tantalizing us with the implication that there is something important about Selim’s blood (meaning his family relationships) that Giaffir knows, and Selim does not.
Later in the poem it is revealed that
Selim is actually the son of Giaffir’s brother Abdallah, whom Giaffir murdered. This means that Selim and Zuleika are cousins, not siblings or half-siblings, and their love is not forbidden.