In the sticks
In the country; especially the unsophisticated backwoods.
'Stick' is one of the older words in English. It dates from around the 10th century and was first put into print in Old English Leechdoms, 1150, with the meaning of 'a slender branch or twig of a tree when cut or broken off':
grennne sticcan hæslenne [freshly cut hazel twigs]
In the following thousand years, all manner of thin pointed objects have been called sticks - ships' masts, conductors' batons, cricket stumps, cigarettes, violin bows, French loaves, and so on. As befits such commonplace objects, sticks have made their way into many phrases - 'over the sticks' (horse racing over fences), 'between the sticks' (football goalposts), 'up sticks' (move one's tent'), 'sticks and stones may break my bones' and so on.
'In the sticks' is just a reference to an area where there are lots of twigs, i.e. the countryside. It was first an American expression but is now used throughout the English-speaking world. The earliest citation of it that I have found is from the US newspaper the Florence Times Daily, November 1897:
... he gathered from 1 1/2 acres this year 21 barrels of corn. If any man "away in the sticks" can beat this, in the language of "Philander Doesticks," we exclaim, "let him stand forward to de rear."
For a time, the phrase became specifically associated with baseball. 'The sticks' were exhibition games, played in county locations, which baseball players organised to supplement their income outside the main season. It was not allowed by the rules of the US Baseball Commission, but the rules weren't often strictly applied. The practice was referred to in the Daily Colonist, October 1921:
"Judge Landis has not yet consigned Babe Ruth to oblivion for playing in the sticks for exhibition money."
The best known reference to 'the sticks' in any newspaper was the 'Sticks Nix Hick Pix' headline in Variety, 17th July 1935. This was a famously succinct expression of the opinion that 'people in the backwoods [sticks] aren't interested [nix] in films [pix] about rural [hick] issues'. Four does seem to be just about the minimum number of words needed to express that idea.
See other phrases that were coined in the USA.