late frost; frost in late spring
A seasonal word (季語) for haiku and poetry, referring to frost that occurs after one has almost forgotten about winter, typically in late spring. Often carries a poetic, wistful nuance. Compare with 別れ霜 (わかれじも), which is a synonym.
See also: 別れ霜 (わかれじも)
Surprised by the late frost, I hurriedly brought in the potted plants from the garden.
In haiku, 忘れ霜 is used as a spring seasonal word.
Compound of 忘れ (wasure, 'forgetting') + 霜 (shimo, 'frost'), with rendaku voicing of しも to じも. The exact historical derivation is uncertain, but the word poetically describes frost that appears when one has almost forgotten about winter.