These days, it seems that whenever you get "Tony Blair" in a sentence, sooner or later you’ll hear "lame duck". Here is the origin of this odd expression :

The phrase "lame duck" was first applied on the London Stock Exchange in the 18th century to brokers who could not pay their debts. Beginning in 19th-century America, "lame duck" was used to describe a Congressional representative who had failed to hornswoggle the voters into re- electing him in November, but who was not due, under the Constitution, to actually be booted out until the following March. Thus freed of even the pretence of accountability to the voters, such "lame ducks" usually voted themselves a scandalous jackpot of perks, until a stop was put to the practice by the "Lame Duck Amendment" of 1934.

(from Word detective)
I still wonder: why duck?