(For noun and adj. definition of slovenly woman, and slovenly, it is a dialect form of SLAMMOCKING)
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Of obscure origin; the shorter forms slammock(s, slommack(s occur widely in dialect, but are not recorded before the 19th century.
From E-NED: Mrs. Slammekin, who is described as affecting a careless undress, is a character in Gay's Beggars' Opera (1727). It is more probable that the colloquial word suggested the name than that it was subsequently derived from it.
1756 - The Connoisseur. Number 134;
see Example below
From: The Connoisseur
By Mr. Town, Critic and Censor-General,
Volume the Fourth, 1757.
Numb. CXXXIV. Thursday, August, 19, 1756.
Mr. Village to Mr. Town