axle, axletree / a very heavy rain, cats and dogs (from the idea of raindrops as big around as a cart's axle) / short for 車軸草 (shajikusō, “clover”)

to rain very heavily, to rain cats and dogs (from the idea of raindrops as big around as a cart's axle)

to rust

needed, necessary

need, necessity

exchange; swap; give-and-take / (mathematics) commutation

to exchange

lupine clover (Trifolium lupinaster)

unnecessary

needlessness