It was stupid of me not to have seen at once that he was connected with the diplomatic service. He had all the marks of the profession. He had the supercilious courtesy that is so well calculated to put up the backs of the general public and the aloofness due to the consciousness the diplomat has that he is not as other men are, joined with the shyness occasioned by his uneasy feeling that other men do not quite realize it.
(W.S. Maugham; The Human Element)
Would you be so kind to explain to me what the part in bold is supposed to mean?