Shaw takes his title from Virgil's Aeneid: "arma virumque cano,"which means "of arms and the man I sing." Virgil's Aeneid is the heroic story of the Trojan warrior Aeneas, who is credited with founding Rome.
The title is ironic, for Shaw's play, though not an attack on war, is an attack on romanticizing war. While Aeneas is most notable for his unwavering piousness and loyalty, functioning as a romantic vision of the noble warrior, the Aeneas figure in Shaw's play, Sergius, is a rigid rather than pious figure: he repeatedly (and significantly) stands with his arms folded and refuses to apologize. He's also less than loyal, as he gets involved with Louka, the servant of his fiancee, Raina. He is also shown to be not much of a soldier after all. Despite wanting to be the grand military hero, he represents the reality that lurks under the romance of phrases like "arms and the man." Shaw puns on "arms" in the sense that Sergius is always folding Louka into his "arms" and declaring his ownership over her. Women to Sergius are another field of conquest.
But Shaw was no doubt also aware that beneath the heroic rhetoric, Virgil was also noting that violence leads to more violence. The Trojan War was a bloody mess, in which many lives were wasted, and Virgil alludes to this carnage when he shows that Aeneas is quick to lead his men into futile battle. Aeneas says:
Come, let us die,
We'll make a rush into the thick of it.
On multiple levels, Shaw's title points to falseness and problems that come from romanticizing war and seeing love as "conquest."