The first witness was the Duke of Denver, who claimed to have discovered the body. He gave evidence that he was coming into the house by the conservatory door at three o'clock in the morning of Thursday, October 14th, when his foot struck against something. He had switched on his electric torch and seen the body of Denis Cathcart at his feet. He had at once turned it over, and seen that Cathcart had been shot in the chest. He was quite dead. As Denver was bending over the body, he heard a cry in the conservatory, and, looking up, saw Lady Mary Wimsey gazing out horror-struck. She came out by the conservatory door, and exclaimed at once, “O God, Gerald, you've killed him!” (Sensation.)
The Coroner: “Were you surprised by that remark?”
Duke of D.: “Well, I was so shocked and surprised at the whole thing. I think I said to her, 'Don't look,' and she said, 'Oh, it's Denis! Whatever can have happened? Has there been an accident?' I stayed with the body, and sent her up to rouse the house.”
The Coroner: “Did you expect to see Lady Mary Wimsey in the conservatory?”
Duke of D.: “Really, as I say, I was so astonished all round, don't you know, I didn't think about it.”
The Coroner: “Do you remember how she was dressed?”
Duke of D.: “I don't think she was in her pajamas.” (Laughter.) “I think she had a coat on.”
The Coroner: “I understand that Lady Mary Wimsey was engaged to be married to the deceased?”
Duke of D.: “Yes.”
The Coroner: “He was well known to you?”
Duke of D.: “He was the son of an old friend of my father's; his parents are dead. I believe he lived chiefly abroad. I ran across him during the war, and in 1919 he came to stay at Denver. He became engaged to my sister at the beginning of this year.”
The Coroner: “With your consent, and with that of the family?”
Duke of D.: “Oh yes, certainly.”
The Coroner: “What kind of man was Captain Cathcart?”
Duke of D.: “Well—he was a Sahib and all that.
Sahib means "friend" in Arabic and was commonly used in the Indian Sub-continent as a courteous term in the way that "Mister" (also derived from the word "master") and "Misses" (derived from the word "mistress") is used in the English language. It is still used today in the Sub-continent just as "Mister" and "Misses", and continues to be used today by English language speakers as a polite form of address.
However, in this context it means, "he was a gentleman."