According to Plutarch, it's what a Spartan mother allegedly said to her son as he was leaving for war, "With your shield or on it." One interpretation of the message I heard and haven't seen anywhere else is that if you come home without a shield, you're a coward because you probably threw it and ran. Those shields were damn heavy and you could travel a helluva lot faster without it. In general, Spartans celebrated those who died in battle much more than those who survived and returned home. To me "on it" means that you were carried home dead by your comrades. You died with honor.
"Sayings of Spartan Women: Chapter 5: Section 19" published on by Loeb Classical Library.
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16. Another, as she handed her son his shield, exhorted him, saying, “Either this or upon this.”b
bReferred to Gorgo as the author by Aristotle in his Aphorisms, as quoted by Stobaeus, Florilegium, vii. 31, but it is often spoken of as a regular Spartan custom. Cf., for example, the scholium on Thucydides, ii. 39. Ancient writers were not agreed whether the second half meant to fall upon the shield (dead or wounded) or to be brought home dead upon it. In support of the second (traditional) interpretation cf. Moralia, 235 a, and Valerius Maximus, ii. 7, ext. 2.