Halfdan m. Proto-Germanic *halbaz 'half' + Proto-Germanic *daniz 'Dane, man from Denmark'.
The name of a 5th-6th C legendary king of Denmark, a legendary king appearing in Scandinavian legends, a mythical petty king in Norway, a 9th C king of Vestfold, a 9th C commander of the Viking army that invaded England, and a 10th C joint king of Northumbria.