Fortnightly means happening once every two weeks. A fortnightly meeting, paid fortnightly, a fortnightly magazine.
It is British and Commonwealth; Americans say every two weeks or biweekly, and biweekly is ambiguous, which is precisely why the British word is useful. A fortnight is fourteen nights, from an Old English way of counting time in nights.
1adjectivefortnightlyHappening once every two weeks.
“The book club has fortnightly meetings, on every other Thursday at the library.”
fortnightly is a frequency.
Old English
fēowertīene niht
Middle English
fourtenyght
English
fortnight
English, today
fortnightly
From fortnight + -ly. fortnight comes from Middle English fourtenyght, fourtene nyght, from Old English fēowertīene niht (literally “fourteen nights"; the Anglo-Saxons counted by nights), equivalent to fourteen + night. Compare sennight.
Etymology from Wiktionary, CC BY-SA