Today a word that seems quite a bit strange at first. Literally it (seems to) translate into rice wife, but how do we get flash of lightning from that? An etymology dictionary tells us:

日本語「語源」辞典<学研教育出版>
いなずま【稲妻】
〔意味〕雷光
〔語源〕「稲の夫(つま)」の意。秋に多く発生するため,古くは稲が雷光と結びついて稲を実らせると信じられていたことから。

inazuma
[meaning] flash of lightning
[origin] Literal meaning rice’s lover. It was believed that rice-plants would join with the lightning to produce ripened plants. Inazuma comes from the fact that this phenomenon often occurs during fall.

Similar word with the same origin and etymology for flash of lightning include 稲光(inabikari, rice plant light), 稲交(inatsurubi) and 霊(inatama, rice spirit).

While today the word tsuma is only used for a wife, it had a much broader usage in the past. It once was a more or less general word for either one’s wife or husband, and was also used to refer to one’s lover irrespective of sex. It was written either with the character 妻(female ~) or 夫(male ~).