Etymology From Middle English septenarie, septenary, from Classical Latin septēnārius (“consisting of seven each”), from septēnus (“seven each; seven at a time”) + -ārius (whence the English suffix -ary). With use as noun compare septēnārius (“line of seven feet; unit of seven parts; period of seven years”). By surface analysis, septen- + -ary. Doublet of septenar and septenarius.
1
Consisting of or containing seven.
2
Of seventh rank or order.
... indeed if further evidence than that of bestial and savage counting were required to show that finger-numeration and the quinary system were not primeval, it would be afforded by the development of the senary-septenary system in so many lands.1899 October, W J McGee, “The Beginning of Mathematics”, in American Anthropologist, 1(4), page 657:
3
Lasting seven years; continuing seven years.
Septenary penance.1655, Thomas Fuller, The Church-history of Britain; […], London: […] Iohn Williams […], →OCLC, (please specify |book=I to XI):
Etymology From Middle English septenarie, septenary, from Classical Latin septēnārius (“consisting of seven each”), from septēnus (“seven each; seven at a time”) + -ārius (whence the English suffix -ary). With use as noun compare septēnārius (“line of seven feet; unit of seven parts; period of seven years”). By surface analysis, septen- + -ary. Doublet of septenar and septenarius.
1
A group of seven things.
2
A period of seven years.
This idea was based on the doctrine that a man's body changed its character every seven years and that his life was thus made up of ‘septenaries’.1971, Keith Thomas, Religion and the Decline of Magic, Folio Society, published 2012, page 596: