From IMDB FAQ
Why is it spelled "Basterds"?
Tarantino commented on "The Late Show" that Inglourious Basterds is the "Tarantino way of spelling it," but he hasn't commented on where the idea for the misspelling arose, nor is he likely to. "I'm never going to explain that," Tarantino was quoted at the Cannes Film Festival.
Three theories have been offered by viewers.
Basterd may be derived from the word Baster, a word derived from Dutch bastaard (bastard). The original Basters were mainly persons of mixed descent between the Cape Colony Dutch and indigenous African women who at one time would have been absorbed in the white community. In the movie, the Basterds are American/Jewish, and their plan was very similar...to be "civilians absorbed" in France, walking among the Nazis.
The misspelling may connote that Lt Aldo Raine (Brad Pitt), who grew up in a family of backwoods bootleggers and moonshiners, has had little in the way of schooling. The words are briefly shown in their misspelled form on his rifle.
Tarantino misspelled the title on the cover of the script, which then leaked onto the internet before production began. Rather than admit his mistake, Tarantino chose to maintain the spelling.
The above were merely audience opinions. But here's a promising article from RottenTomatoes where Weinstein Co. and Universal Pictures, confirms that QT is not a poor speller and gives the reasons for the title.
Inglourious Basterds begins in German-occupied France, where Shoshanna Dreyfus (Mélanie Laurent) witnesses the execution of her family at the hand of Nazi Colonel Hans Landa (Christoph Waltz). Shosanna narrowly escapes and flees to Paris, where she forges a new identity as the owner and operator of a cinema.
Elsewhere in Europe, Lieutenant Aldo Raine (Brad Pitt) organizes a
group of Jewish soldiers to engage in targeted acts of retribution. Known to their enemy as "The Basterds," Raine's squad joins German actress and undercover agent Bridget Von Hammersmark (Diane Kruger) on a mission to take down the leaders of The Third Reich. Fates converge under a cinema marquee, where Shosanna is poised to carry out a revenge plan of her own...