Dictionary.net

• The Norse name of those piratical leaders whose ravages on the British coasts began in the ninth century; sea-kings were properly those connected with a royal race, or the small kings of the country.

1874 - Etymological and pronouncing dictionary of the English language
By Stormonth, James, Phelp, P. H.

• [Icel.] The Norse name for the Sea-kings, whose assaults on this country began in the ninth century.

1895 - Glossary of terms and phrases
By Henry Percy Smith