The Multnomahs were Native American people who lived downriver around present-day Sauvie Island
We frequently see "multnomah" referenced in place names like Multnomah Falls (natural site), Multnomah Village (neighborhood), and Multnomah County (principle county in Oregon and home to Portland).
But the word didn't originate from any of these. "Multnomah" was actually a Native American word that referred in a broad sense to the people and tribes living on or near today's Willamette River. Generally, the word is thought to have meant 'down river' and referred widely to the tribe, the village, the chief, and the large island that sits at the confluence of the Willamette and Columbia Rivers, now called Sauvie Island.
Interestingly, the word was first recorded in the journals of explorers Lewis & Clark in 1805, where they spelled it "Mulknomah." In 1806, this tribe's estimated population was close to 800, but by 1835, disease is said to have devastated this tribe.