the over-under is....

[George Thompson]

"Over-under" is a sports gambling term not in the recently revised OED.

Searching the Proquest newspapers for "over under" turned out to be unprofitable, since that sequence in other contexts is far more common than I would have thought.  Searching for "over under is" takes it back to September 24, 1979, in the Chicago Tribune:
Latest line.  Favorite Dallas, Points 3 Underdog Cleveland.  Over-Under: 39  Over -Under is the betting line on the total number of points scored by the two teams in the game, over or under the line figure.

In the LA Times, December 28, 1980, Jim Murray made plans to teach his new granddaughter that "over-under is a sucker bet", along with a number of other bits of no doubt hard-earned wisdom.

By 1987, Tony Kornheiser was using it in the Washington Post in other contexts, as for instance, the Washington Redskins line coach favors very big men, with 300 lbs as the over-under.  (not a verbatim quote).
No doubt the old wisecrack regarding an ill-matched marriage, "I give it six months" is now, in some circles, being expressed as "I put the over-under at six months".  (Meaning no application to any marriage recently in the news.)

GAT

George A. Thompson
Author of A Documentary History of "The African Theatre", Northwestern Univ. Pr., 1998, but nothing much lately.  Working on a new edition, though.

------------------------------------------------------------
The American Dialect Society - http://www.americandialect.org