Uh, well then, you're not nearly as nerdy as Daily RFT. During a recent game, we came up with four possibilities:
1. The team's early owners were ardent birdwatchers and especially fond of the northern cardinal, state bird of neighboring Illinois.
2. The nicknames Red Stockings (or Sox) and plain old Reds were already taken.
3. The name was a tribute to the overwhelming Catholicism of the citizens of St. Louis.
4. Unfortunately, the team was founded more than a century before the inaugural appearance of the Rally Squirrel. (St. Louis Squirrels has a certain ring to it, no?)
But then we decided to put down our beer and do a little research.
In 1899, the St. Louis National League team was known as the Perfectos. This was the result of a major effort in what is now known as "rebranding." In its previous incarnation as the St. Louis Brown Stockings, the team had finished the 1898 season with a 39-111 record. Also, part of the League Park, where the Browns played, had burned down and owner and sometime-manager Chris von der Ahe declared bankruptcy. He was also kidnapped by bondsmen for failing to pay his debts and divorced by his wife. Truly, 1898 was not his best year.
In 1899, Frank and Stanley Robison, who owned the Cleveland Spiders, took over the St. Louis franchise and transferred many of their stars, including Cy Young, to the new team. (The 1899 Spiders set a new major league record for futility: They finished the season at 20-134 and subsequently got kicked out of the National League.) They also changed the team's uniform to white with red trim with red-and-white-striped stockings.
Later that summer, Willie McHale, a columnist for the St. Louis Republic, reported that he'd overheard a lady in the stands pronounce the new uniforms "a lovely shade of cardinal." McHale was, presumably, so charmed by the lady's grasp of color names that he began referring to the team as the "Cardinals." It was, after all, an age when teams were regularly -- and unimaginatively -- named after the colors of their uniforms (not only the aforementioned Brown Stockings, Reds and Red Sox, but also the Chicago White Stockings, Toledo Blue Stockings and Providence Grays) and "Perfectos" wasn't very descriptive of the team's record. Also, it was sort of lame. So was an alternative nickname, Tebeauites, after team manager Patsy Tebeau. (It was just as well "Tebeauites" never caught on; Tebeau resigned halfway through the 1900 season.)
We don't actually have any record of McHale's encounter with the female fan, but we've found the story corroborated by three different sources, including the Baseball Almanac and the Cardinals' own website, so we're going to go with it.
By the opening of the 1900 season, the nickname had fully caught on. On April 28, the rival Post-Dispatch made its first reference to "Cardinals":
And yes, the uniforms were, indeed, still cardinal:
Management didn't start thinking of "cardinals" in terms of birds until the 1920s when it adopted the first incarnation of the birds-perched-on-a-bat logo and put it on the uniforms. It would take almost as long for the team to exorcise the bad Brown Stockings mojo: It didn't post a winning season until 1911 and didn't win a National League pennant until 1926.
The Texas Rangers were named after the legendary frontier police force by their then-owner, Robert Short, in 1972 when they moved to Texas from Washington, DC. (Mental Floss has an entertaining rundown of team name origins. And here's a visual representation.)
Incidentally, the football Cardinals have nothing to do with the baseball Cardinals. The football team originally played in Chicago and acquired its nickname because its owners were too cheap to order new uniforms. Instead they purchased them secondhand from the University of Chicago Maroons and figured maroon and cardinal were close enough.