noun
Archaic familiar term used by men to address a wife, lover, or close female friend. Not used in modern Japanese outside historical or poetic contexts.
古歌に「吾妹が寝ぬる夜の雨」とある。
An old poem says, 'The rain on the night when my love cannot sleep.'
From Old Japanese 我 (wa, 'I, my') + 妹 (imo, 'beloved woman, sister'). The exact historical derivation is uncertain, but it is a conventional archaic compound.