Helmet – although seemingly unrelated at first sight, the word is actually a cognate to the word Hell. And surprisingly, even the name for the Greek nymph, Calypso, is related to the English Hell, too.
On one hand, Helmet is a diminutive word of the Middle English Helm, a helmet or defensive cover for the head. Helm comes from the Proto-Germanic *helmaz, a protective covering.
On the other hand we have Hell, which comes from the Old English Hell, Helle and has the meaning of a place of torment for the wicked after their death. It derives from the proto-Germanic word *haljō, the underworld, a concealed place.
Both words have a meaning of covering or concealing something and they are believed to derive from the Proto-Indoeuropean root *kel-, which would have conveyed the meaning of covering, concealing and/or saving.
Since English is a Germanic language, we can find many cognates to Hell and Helmet across basically all other Germanic languages. Outside of these, however, there is a Greek word that also derived from *kel- that I would like to mention – the Greek name Calypso.
Calypso is a mythological figure that appears in the Odyssey. She trapped Odysseus to keep him as her husband on her island for seven years, until he escaped.
Her name comes from the Greek verb kalyptō, which very fittingly means to cover, conceal, hide and even to deceive. This verb also derives from the Proto-Indoeuropean *kel-, making it a cognate to Helmet and Hell in English.
This shows us again, how some words might still be related in their origins and meaning to some pretty unexpected words in other languages. Pretty amazing, if you ask me.