But there are other possibilities to consider, too.
At the time Key was writing, the word “slave” (we’ll get to “hireling” in a minute) had long functioned in English as a wide-ranging epithet, hurled at persons of any and all colors, nationalities, and conditions of servitude or otherwise.
Shakespeare, who barely mentioned America in his writings, used the word more than 180 times in his works. Fewer than a third of those references are in the plays set in Roman and Greek times, in which characters in the drama might be literal slaves. More often, Shakespeare’s characters — including Macbeth, Lear, and many of the kings in the history plays — use “slave” as an insult. (“O what a rogue and peasant slave am I!” says Hamlet, a prince, as part of a self-lacerating soliloquy.) Though often signifying low birth, these uses have no connection to color or — aside from the frequent use of the epithet to put down the French — to nationality. Still less do they connect to the institution of chattel slavery as found in the Americas.
This usage had not disappeared by Key’s lifetime. In Robert Burns’s battle poem “Scots Wha Hae,” written in 1793 though set more than 400 years earlier, the word “slave” is an insult directed at his fellow Scots who would flee rather than follow their king into the Battle of Bannockburn.
To Americans, while “slave” was both a common descriptive word and an epithet, “hireling” — especially in contexts of poetry and literature — ordinarily carried derogatory connotations. It meant someone such as a soldier, official, or laborer who served for money rather than from some more durable loyalty such as to family or nation. Yet another Robert Burns song, “Parcel of Rogues,” describes Scotland as having been sold out for “hireling traitor’s wages.” “Hireling and slave” is not an accidental pairing; the two words often occurred together as epithets.