The story
The earlier spelling "cohose" was re-interpreted as a plural form (for a similar development, see pea). From Halkomelem.
Etymology from Wiktionary, CC BY-SA 4.0.
coho
Today it means: An anadromus and semelparous salmon, Oncorhynchus kisutch, found in the coastal regions of the northern Pacific Ocean, used as a symbol by several Native American tribes. Full definition →