Vine's Expository Dictionary of NT Words
Eight, Eighteen, Eighth
"eight" (Lat., octo, octavus; cp. Eng., "octagon," "octave," "octavo," "October," etc.), is used in Luke 2:21; 9:28; John 20:26; Acts 9:33; 25:6; 1 Peter 3:20; in composition with other numerals, okto kai deka, lit., "eight and ten, eighteen," Luke 13:4,11,16; triakonta kai okto, "thirty and eight," John 5:5 .
an adjective, signifying an "eighth-day" person or thing, "eight days old" (okto, and hemera, "a day"), is used in Philippians 3:5 . This, and similar numerical adjectives not found in the NT, indicate duration rather than intervals. The Apostle shows by his being an "eighth-day" person as to circumcision, that his parents were neither Ishmaelites (circumcised in their thirteenth year) nor other Gentiles, converted to Judaism (circumcised on becoming Jews).
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Vines, W. E., M. A. Entry for 'Eight, Eighteen, Eighth'. Vine's Expository Dictionary of NT Words. https://www.studylight.org/dictionaries/eng/ved/e/eight-eighteen-eighth.html. 1940.