It is bender.
The origins of this term remain, appropriately enough, hazy. We do know that the drinking spree meaning of "bender," which first surfaced in the mid-1800s, derives from a slang use of the verb "to bend" to mean "to drink heavily."
Some say "bend" refers to the bending of the elbow that occurs while lifting a glass or bottle to one's lips. Others claim that "bend" and drinking became connected because one meaning of "bend" is "to exert great effort," as in "to bend to the task"; someone engaged in an energetic bout of drinking was perhaps seen to be "bending to the flask ... er, task."
A third explanation for this coinage is, well, a coin. An obsolete British coin, the sixpence, was known as a "bender" because, being made out of silver, it could be bent. In the old days, a bender could buy you enough booze to stay drunk for quite a while, so the resulting binge came to be called a "bender." But my sixth sense tells me this six cents derivation is phony; there's not a dime's worth of evidence to support it.
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