It could come from the word hís which means "the cut" (possibly referring to how the river Nidelva turns near the island).

[7] On 19 July 1930, a royal resolution changed the spelling of the name of the municipality to Hisøy.

The official blazon is "Gules, two lighthouses argent issuant from the base" (Norwegian: I rødt to oppvoksende sølv fyrtårn).

This means the arms have a red field (background) and the charge is two twin lighthouses.

The lighthouses have a tincture of argent which means they are commonly colored white, but if the arms are made out of metal, then silver is used.