"devious, shady, illegal," 1640s, from back + door. The notion is of business done out of public view. The association with sodomy is at least from 19c.; also back-door man "a married woman's lover," black slang, early 20c.
an arrangement of five objects, as trees, in a square or rectangle, one at each corner and one in the middle.
a screen or mat covered with a dark material for shielding a camera lens from excess light or glare.
a printed punctuation mark (‽), available only in some typefaces, designed to combine the question mark (?) and the exclamation point (!), indicating a mixture of query and interjection, as after a rhetorical question.
a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.