I googled "Poker Dictionary" and eventually found one, written by Michael Weisenberg for Planet Poker's website (remember them?).
straddle
1. (v) overblind (definition 1). 2. (n) overblind (definition 2). Someone might say, "John acts last; he has the straddle." 3. The second of two forced blinds, usually put in by the player two positions to the left of the dealer position. In former years, these two bets were called blind and straddle. Some say that definition 3 is the only proper use of the word straddle.
In definitions 1 and 2 above, the word "overblind" links to this definition:
overblind
1. (v) Put in a blind when one is already present. In a traveling blind game, this could mean someone putting in an optional blind in addition to the mandatory blinds. In a game without mandatory blinds, this would be blinding a pot (putting in a blind) after someone else has killed it. (To put in an overblind is sometimes called to kill.) Sometimes called go the overs. 2. (n) The blind put in by the person who overblinds. In a 3-3-6 traveling blind game ($12 limit or $12 minimum bet no-limit), John might put in $12 before getting his cards. He has doubled the limit (or the minimum bet) to $24, and he gets last action before the draw. Someone might say, "John acts last; he has the overblind." Also straddle, for both meanings.
That definition had a link to "Traveling Blind Game", but we'll be here all day if I keep copying links.
It may not spell out the origin, but it at least sheds some light.
http://www.planetpoker.com/games/dictionary/