Despite the initial reaction this expression is not used to literally suggest that someone has been dragged backwards through a thorny or jaggy hedge. Rather, it is used in a less literal context.
The expression is used to express the idea that someone looks a mess. In general the expression is most often employed to suggest that someone’s hair is messy and thus it looks as though they have been dragged through a hedge backwards despite the fact that in reality this is not the case.
This expression is predominantly used in Britain and this is because it originates from 19th century England. The first recorded usage of it can be found in the 1857 edition of The Hereford Journal which reads “in the class for any distinct breed came a pen of those curious birds the silk fowls, shown by Mr. Churchill, and a pen of those not less curious the frizzled fowls, sent by the same gentleman, looking as if they had been drawn through a hedge backwards.” This use of the expression was related to the way in which the chickens being sold looked rather than the more common use nowadays which relates to human beings.
The phrase has been revived more recently and has begun to be used a lot more commonly thanks to English chef and food writer Hugh Fearnley-Whittingstall who promotes foraging in hedges to find food and thus the literal sense of the phrase is beginning to be closer to the truth. He is quoted to have said “if I had a pound for every reviewer who said I looked as if I’d been dragged through a hedge backwards, I’d have, ooh, about 17 quid minimum. Actually, I have never been dragged through a hedge, backwards or forwards.”