[A fanciful and fake etymology from my series Devil’s Derivations: Etymologies from Hell (found here: https://paulwhitberg.wordpress.com/fun-with-etymology-devils-derivations/)]
Although no longer used as a compound, “bargain” was originally formed in the early 1700s by combining “bar” and “gain”. While the meaning of “gain” is clear, the relevant sense of “bar” is less certain. Two prominent theories disagree about the correct meaning of “bar”: one theory suggests that “bar” refers here to the legal term associated with lawyers (or “barristers”), while the other theory asserts that the relevant “bar” is the one at which alcohol is served.
Both theories agree that “bargain” referred originally to an agreement disadvantageous to a swindled party and that “bargain” gradually shifted in denotation and connotation from the negative “that which is unjustly obtained at a reduced price” to the more positive “that which is obtained at a reduced price”. The “Legalists”, as they call themselves, argue that “bargain” originally referred to the type of deals sly lawyers managed to obtain for clients. If a clever person swindled someone else, people would say that the beneficiary had gained the sort of deal usually obtained only by “the bar”—a shortened form of “members of the bar”, i.e. lawyers. “Alcoholics,” on the other hand, insist that “bargain” was the sort of “gain” a person might obtain from someone who had been drinking heavily at a “bar;” the Alkies—as many know the group—has found evidence that con artists would pace their drinking carefully to remain more or less sober while befriending drunken people at bars. The con artists would then convince these drunken people to sell or exchange valuables at low prices and coax the intoxicated dupes into making imprudent bets and investments. A successful con would often brag that he or she had walked away with a “bar gain”—i.e. the type of gain one was mostly likely to obtain at a bar.*
*Regardless of the term’s origins, the Alkies’ explanation is the one that most early moderns accepted. Supporting the Alkies’ claims is a well-known example from Anaximander Snope’s An Essay on Witticism:
When fellows get themselves a little drunk,
Transactions smelling foul as any skunk
Obtain with ease their signed and sealed consent
[A fact for which they later will repent]—
…For, while the lads are drinking to my health,
I’m busily depriving them of wealth.
The morning after, dupes will blame their stars,
And I will count the gain I got from bars.