The story
From slap + -er. For senses 2 and 3, the OED tentatively quotes the Bloomsbury Dictionary of Contemporary Slang: "This working class term from East London and Essex is probably a corruption of shlepper or schlepper, a word of Yiddish origin, one of whose meanings is a slovenly or immoral woman."
Etymology from Wiktionary, CC BY-SA 4.0.