Horace LaBadie explained on 11/24/2013 :
But, like Jerry, I suspect a phrase-out-of-time (although I won't
provide cites as good like his). And like Jerry, I've associated it
with self-trained and often do-it-yourself mechanics; there's a show
somewhere on cable called "Shadetree Mechanic", and my impression from
seeing it during channel surfing is that it's audience is
do-it-yourselfers.
"Armchair detective" would be another anachronism if it appeared in
this book, but "jailhouse lawyer" goes back at least as for as
Hollywood Westerns.
/dps
--
"This is all very fine, but let us not be carried away be excitement,
but ask calmly, how does this person feel about in in his cooler
moments next day, with six or seven thousand feet of snow and stuff on
top of him?"
_Roughing It_, Mark Twain.