The story
Coined by Terry Nation in 1963 for his script of The Mutants (later retitled "The Daleks"), the second serial of the first season of Doctor Who. Accounts of how he devised the word vary. Within the Doctor Who universe, the word is supposed to be related to the name of the people from whom the Daleks evolved, either the Kaleds (as according to later serial Genesis of the Daleks) or the Dals (as in The Daleks), or to derive from a word meaning "gods" in their (fictional) language, in which the sacred "Book of Predictions" is written. According to the I, Davros audio fiction the book reads "...and on that day, men will become as gods", with the last word pronounced "Dal-eks".
Etymology from Wiktionary, CC BY-SA 4.0.