The Icelandic word "geysir" refers to a specific geyser in Iceland called Geysir, which is actually the most famous geyser in the world. This geyser was named after its characteristic gushing action, and the term subsequently became the generic term for all such hot springs.
So, in essence, the word "geyser" was born from the specific name of a famous hot spring in Iceland, reflecting its most defining characteristic - its forceful, gushing eruption.