Going down a Wikipedia rabbit hole:

[Portland, Maine] was formally founded in 1786 and named after the English Isle of Portland. In turn, the city of Portland, Oregon, was named after Portland, Maine.

I failed at finding how the Isle of Portland got its name but saw this:

In Dorset, England:

The origin of the name “Portland” on the Isle of Portland is uncertain, but theories include:

• It may be a corruption of the Celtic word “Port Lann” (“harbor by the cliff”).
• It may derive from the Old English “portelond” (“land by the harbor”).
• It may refer to a fortified harbor or headland.

etymologyworld.com/item/portland

Its the first time I’ve seen the site though and that page feels a bit AI generated