Spilled or splashed; covered with slopped liquid; in a sloppy or messy state.
From 'a-' plus 'slop,' which has murky origins—possibly from Old English 'sleopan' (to slip) or influenced by the verb 'slap.' By the 1600s it meant spilled liquid or mud, and 'aslop' emerged as the adverbial form.
Imagine saying 'the deck was aslop with seawater' instead of 'the deck was covered with sloshed seawater'—sailors probably used this word, and it's a great reminder that English had much more colorful adverbial forms before we flattened our grammar.
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