Once hyphenated and twice not? This apparent contradiction arouses my curiosity!
- 06.067 down the hillsides from their gate
- 06.086 and there with the light of the moon on a hill-side rock or a stream
- 09.018 from the hillside there was a water-gate.
To my surprise, the hypenated or two-word form are attested in OED, the single-word form is not… until it is itself one member of a combination word
1890 Daily News 20 Dec. 5/6 The name ‘Hillside men’..applied to the Fenians.
“hill-side, n.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/87009. Accessed 14 September 2017.