Women are few and far between in Lord Jim, which means we're reading a novel that's all about men being men – sailing, pirating, and fighting (okay, the ladies get in on a little of that action, too). Being a man in the British Empire was all about acting with a sense of duty and honor, and when Jim fails to do so, he runs into all kinds of problems and loses the respect of just about everyone he knows. Yet there are hints throughout the novel that Jim is not some sort of strange, unmanly anomaly. The more characters like Marlow relate to Jim, the more that calls into question the standard ideas of masculinity at the time.
In Lord Jim, being manly means behaving with a sense of duty and honor. For much of the novel, Marlow sees Jim as unmanly because of his lack of duty and honor, but when Jim willingly goes to his death at the end of the novel, he regains his gentlemanly status.
We can read Lord Jim as the story of Jim's development from boyhood to manhood, which he only gains by facing his death at the end of the novel.