Probably prolonged from hals; the sea (genitive case or specially) -- sea.

see GREEK hals

θάλασσα (thalassa) − 6 Occurrences

θάλασσαν (thalassan) − 43 Occurrences

θαλάσσῃ (thalassē) − 13 Occurrences

θαλάσσης (thalassēs) − 29 Occurrences