(archaic) 'the sixth month of the lunar calendar'

From Old Japanese, a compound of 水 (mi, “water”) +‎ な (na, alternative form of possessive particle の (no)) +‎ 月 (tsuki, “month”). Originally referred to the month in which rice paddies need more irrigation; the use of 無 (“not; none; without”) is in reference to this need for more water.