It appears that the word was coined in 1887; while there doesn't seem
to be much concrete evidence or "story" behind the origin of the word
available, it is quoted as originating in 1887 in the Merriam-Webster
I believe this term to be in common use, at least throughout English
speaking countries, of course; I've heard it on any number of
television shows (primarily sports events, of course, where the term
is used more often than in an everyday conversation.) The cartoon
character mentioned in the comments is more likely to have originated
the "playground" usage of the word, but the cartoon character was in
turn named after a term created over a century ago as.. well, a
synonym for "loser." :)
(As for how commonly the "underdog" thing is on elementary school
playgrounds.. I remember the term myself. I imagine it was quite
common among Generation X kids, but I imagine usage has dropped
drastically with the advent of newer, louder, more obnoxious kids'
fads. Pokemon, and what-have-you. :)
Hope this helps!
From Princeton University's "WordNet" (parsed by dict.org):
n : one at a disadvantage and expected to lose
From an "Online Etymology Dictionary":
[Sources quoted on http://www.geocities.com/etymonline/ for this
listing of words]
underdog - "the beaten dog in a fight," 1887.
From Merriam-Webster's Collegiate Dictionary:
One entry found for underdog.
Main Entry: unĚderĚdog
1 : a loser or predicted loser in a struggle or contest
2 : a victim of injustice or persecution
Search terms used:
etymology underdog [google]