One who still doesn't quite have the local terrain, weather, driving skills, et cet. down. This looks like a word derived from a First Nations language, and indeed it is, but not from that area -- it's Chinook Jargon, which developed as a trading pidgin in the lower Columbia River valley and spread throughout the Pacific Northwest. It died out shortly after WWII, but words from it are still in use throughout the area, especially British Columbia -- this one surviving only further up north. Like most pidgins, Chinook Jargon took vocabulary from several languages, but this one actually comes from Chinook itself, at least partly: from chee, new (from Lower Chinook čxi, new/right away) + chako, come, from Nootka (a Vancouver language) čokʷa·, come (imperative form). First recorded use as an English term is from 1897, apparently during the Yukon Gold Rush. For an example, Jack London:
The cold white silence of the Yukon Wilderness plays savage tricks on the minds of cheechako and sourdough alike.
(Bonus word: In Alaskayukon, a sourdough is a permanent resident -- one who's lived there all four seasons of the year.)