I have heard two theories about why police officers were once referred to as “Flatfoots”.
One is that a cop on the beat would develope flat feet from walking a beat day after day. I’ve found at least one dictionary that cites this as the origin.
The other theory (the one I favor) speculates that a job in law enforcement would appeal to those that were not accepted for military service. One common reason for being declined by the Army was flat feet.
If the term is a derogatory one, I think the second theory is more likely. The conotation that a cop was “washed out of the Army” seems more insulting than taunting him because he has to walk a lot.
Does anybody have any evidence that supports or disproves either of these theories? Or perhaps a totally different etymology?