From hune(“giant”) +bed(“bed”), roughly meaning "a bed (of stones) built by giants". Cognate to and probably formed after GermanHünenbett(“dolmen”).
The alternative form hunnebed(“dolmen”) also exists, which is a conflation with the word Hunnen(“the Huns, an Asian tribe which lived in Europe and Central Asia between the 1st century AD and the 7th century”). As a consequence preference is given to the etymologically correct hunebed.