"Sumas" is an Indian word meaning "a big level opening". Sumas Lake, once extensive, shallow and infested with mosquitoes, has been drained for many years, providing lush agricultural land. The earliest mention of Sumas Mountain is to be found in Simon Fraser's account of the country he passed through on June 30, 1808: "...continued our course with a strong current for nine miles [down the Fraser] where the river expands into a lake. Here we saw seals, a large river [the Chilliwack River ?] coming in on the left, and a round Mountain a head [sic], which the natives call shemotch." In the mid-19th century Sumas was often spelt "Smess".
Source: BC place name cards, or correspondence to/from BC's Chief Geographer or BC Geographical Names Office