Green’s Dictionary of Slang

show-leg day n.

also shulleg-day
[on such days women accidentally/are forced to raise their long skirts and thus ‘show a leg’, or at least an ankle]

1. a very windy day.

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[UK]Partridge DSUE (1984) 1063: [...] 1887; ob. Baumann.

2. a very muddy day.

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[UK] (ref. to 1880) in J. Ware Passing Eng. of the Victorian Era.
[UK]Partridge DSUE (1984) 1063/2: ca. 1880–1925.