sennight means synonym of week in its various senses, a period of seven consecutive days and nights. Lexicurio rates it Sui generis — a strength score of 87 out of 100.
sennight is pronounced /ˈsɛnaɪt/.
Why “sennight” is a great word
SENNIGHT — [Noun] A period of seven consecutive days and nights; a week. From Middle English senight, a contraction of seven-night, from Late Old English seofeniht, from seofon ("seven") + niht ("night", used in reckoning time). Unlike "fortnight," which telescopes two cycles of seven nights, or the plain "week," which has shed its nocturnal count, sennight roots the calendar in the turning of the earth from darkness to darkness. It is the slow unfurling of a moon from sliver to half, the patient tally of shadows crossing a cottage floor, and the agreed-upon span between one market day and the next—a word that quietly insists time is felt, not merely counted.
Etymology
From Middle English senight, senyght, sinight (“sevennight, a week”) [and other forms], a shortened form of seven-night, sevenight, sevenyght [and other forms], from Late Old English seofeniht (compare seofonnihte (“seven days old; seven days long”, adjective)), from seofon (“seven”) (ultimately from Proto-Indo-European *septḿ̥ (“seven”)) + niht (“night; day (when computing spans of time)”) (ultimately from Proto-Indo-European *nókʷts (“night”)). Doublet of sevennight.
noun
- Synonym of week in its various senses, a period of seven consecutive days and nights.“I dranke not this sennet
A draught to my pay;
Elynour, I thé pray,
Of thyne ale let vs assay, […]”