Probably from hapas; one (or a single) time (numerically or conclusively) -- once.
see GREEK hapas
a. universally: 2 Corinthians 11:25; Hebrews 9:26; 1 Peter 3:20 Rec.; ἔτι ἅπαξ, Hebrews 12:28f; ἅπαξ τοῦ ἐνιαυτοῦ, Hebrews 9:7 (Herodotus 2, 59, etc.).
b. like Latinsemel, used of what is so done as to be of perpetual validity and never need repetition, once for all: Hebrews 6:4; Hebrews 10:2; 1 Peter 3:18; Jude 1:3, 5.
c. καί ἅπαξ καί δίς indicates a definite number (the double καί emphasizing the repetition, both once and again i. e.) twice: 1 Thessalonians 2:18; Philippians 4:16; on the other hand, ἅπαξ καί δίς means (once and again i. e.) several times, repeatedly: Nehemiah 13:20; 1 Macc. 3:30. Cf. Schott on 1 Thessalonians 2:18, p. 86; (Meyer on Philippians, the passage cited).