Center Fielder (1862)

“C.F., center fielder” (in list of abbreviations following a pair of box scores). “Base Ball,” Rocky Mountain News (Denver), April 29, 1862, p3
— The list of abbreviations is preceded by this text: “For the benefit of those are not posted on the contractions pertaining to a description of the games, we give them as follows:”

1862 box score abbreviations in Rocky Mountain News

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1866.  “The Center Fielder should always be in readiness to back up the second base, and should always go to long field in cases where a hard-hitter is at bat.” (Henry Chadwick, The Base-Ball Player; Peter Morris).