I found a couple of different explanations:

1) "Deuce" (two) is a slang term for "the Devil".

2) A two is the lowest card in a deck. Gamblers apparently associated the two dollar bill with the "two" card.
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Well, well, well. Sounds like two dollar bills could very well be unlucky. Hey! Anyone out there need to change your luck? If you'd like to rid yourselves of the cursed currency, I can help. I'll send you my address and you can send those godforsaken, devil bills to me. I'll dispose of them in a safe and productive manner.

Between the years 1920 and 1950, the two dollar bill was prevelant. All barbershops were historically closed on mondays, a good day for a lot of barbers to go to the racetrack. Racetracks also paid off with two dollar bills. When the day was over and the barbers went home, if they forgot to remove the two dollar bills, and their wives saw a two dollar bill in their wallets, the fights began. The wife would know the husband had been at the track. So barbers considered them unlucky and that superstition has just passed forward.

Waitress's consider them unlucky...means a poor tip day... when i get them I give to boss dont want them in my pocket

I am sorry but I am leaning towards who cares.
hurtzsogood

An interesting bit of $2 trivia. When Touchstone Picture was filming portions of "Dead Poets Society" at St.Andrews School, Middletown DE in 1988, the production crew were paid exclusively in $2 bills... to demonstrate the effect that production had on the local economy.

I was told way back when when politicians wantyed to buy someone's vote on election day they paid that person $2.00. so when the people went to the bar and paid with a $2.00 bill it was assumed they sold their vote

hey Guys its not Unlucky its a fortune these days 2 dollar bill is equal to 10 $ because its unique & antique everyone is ready to buy that from u if u guys have these bills with u please contact me I live in Dubai and u can email me at karim555@hotmail.com or call me at +97150 6806616
Karim

i dont know. i've never heard that one. i've been taught that the 2 dollar bill is lucky bc its so unique.

I was told it is lucky.

Well, anyone that has some of this unlucky currency can send it to me and I will take the bad luck and dispose of the currency for you. I will only charge you $1.00 to do it! That's right! If you mail me your unlucky $2.00 bills to 1597 Alamo Dr., Vacaville, CA. I will send you $1.00 for each and every one... Allow 4 to 6 weeks for delivery.