From Scout.com:
Probably the two oldest odd names date back to
the sport’s pre-league period, when Eckford played in Brooklyn and
Athletic played in Philadelphia. (Note that the singular nickname
phenomenon did not start in the late 20th Century, it started in the
1850s, when basically all nicknames were singular.) ... The Athletic moniker, the longest-lasting name in
baseball, dates to the spring of 1860 when members of the Handel and
Haydn singing society of Philadelphia decided to form a baseball team.
Although there are a couple of versions of how it happened, basically
one member suggested that the team needed to have a strong name,
something athletic. And thus was born a name that still exists, 148
years later.
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