Late 16th century: from Greek heptagonon, neuter (used as a noun) of heptagonos 'seven-angled'.

• seven from Old English:

  Seven days of the week, seven deadly sins, seven dwarfs, seven wonders of the world, the Magnificent Seven…the number seven crops up again and again in history and culture. The word is from the same ancient root as Latin septem and Greek hepta ‘seven’—septem is the source of September (Old English), originally the seventh month of the Roman year, and of septet (early 19th century), whereas hepta is found in words such as heptagon (late 16th century) ‘seven angled’, and heptathlon [1970s] combined with athlon ‘contest’. The seven-year itch was originally a semi-proverbial medical condition, dating from the middle of the 19th century, but these days is a supposed tendency to infidelity after seven years of marriage. The phrase originated in the USA, and The Seven Year Itch, a 1955 comedy written and directed by Billy Wilder and starring Marilyn Monroe, introduced it to a wider audience.