Critics are basically in agreement that there are three straightforward interpretations of the title, though there are many different ways to analyze the novel in terms of those interpretations. The first two interpretations stem from the dual meaning of the word "arms." "Arms" refers to the "arms" or weapons used in wars, and to the embracing "arms" of human beings. There is also a poem, written by George Peel in the 1500s, called "A Farewell to Arms." (Check it out here.)
Although it has become a commonly accepted "fact," it is only assumed that Hemingway consciously borrowed his title from the George Peele poem. Nobody knows for sure if he even knew the poem existed. Critics note that the poem shows the poet undergoing a series of transformations, somewhat similar to those experienced by Frederic Henry.
If Hemingway did refer to the poem intentionally, it’s an ironic homage for sure. The poem was written to Queen Elizabeth and is a lament that the poet can longer serve the queen by fighting in battles. Unless the poem is ironic as well, we don’t think this is quite what Hemingway had in mind. While the novel presents a romantic view of war, in that the fact of war intensifies the characters’ relationships, some of the characters wound themselves in terrible ways to keep from facing the horror of the battlefield. The fighting itself is not romanticized, but instead presented starkly and realistically as something nobody in their right mind would be sad to stop doing.
Now let’s look at a few ways we can analyze the title using the word "arms." Since the book spends much time describing the horrors of World War I, the title can be seen as a plea to the world to say "farewell" to wars and the weapons we use to fight them. Since Frederic leaves his post as ambulance driver for the Italian army during the retreat, and then flees with Catherine to Switzerland to avoid being arrested for desertion, the title can refer specifically to Frederic’s "farewell" to the weapons of war when he decides to end his personal involvement with it. There is some irony at work here, because when Frederic says "farewell" to the Italian army, he also says hello to the lover’s arms of Catherine. (There are those two meanings of "arms" again.) When she dies, he must say farewell to those arms as well.
Catherine’s death also parallels the death of a soldier in battle. Remember this famous scene? When Frederic is transported in the ambulance, the wounded soldier above him has "hemorrhaged" and the blood drips on Frederic. Catherine too dies of "hemorrhage," though her wound comes from her battle with childbirth.
Looking at the parallel between Catherine and the soldier helps us remember how much love exists between Frederic and the men he encounters in the war. In addition to saying farewell to Catherine’s loving arms, Frederic says farewell to the loving arms of many of these men in the novel. Also, because the novel is written in the past tense as Frederic’s memory of both World War I and of Catherine, the title can be a comment on the paradoxical way that Frederic is dealing with the trauma of both such experiences.
To deal with such intense pain and loss, he relives it through remembering it and telling it. By preserving the events in a narrated memory, he can try to say "farewell" to the arms of pain that bind him, and perhaps make things hurt a little bit less. He also honors the loving arms behind the pain by giving them a place in remembered history.