With most phrases and sayings the meaning is well understood but the origin is uncertain. With this one the main interest is the doubt about the meaning. So, this time, we'll have the origin first.
Origin
'Ne'er cast a clout till May be out' is an English proverb. The earliest citation is this version of the rhyme from Dr. Thomas Fuller, Gnomologia, 1732, although it probably existed in word-of-mouth form well before that:
"Leave not off a Clout Till May be out.
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The earliest known version of this proverb, dated 1732, is "Leave not off a Clout, till May be out". "Clout" here clearly has to refer to clothes, not clods - how would you "leave off" a clod? It's significant that the first version of the proverb in English occurs several decades after the first publication of the translation of the Spanish version in Stevens' "Spanish & English Dictionary" in 1706.
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