Unclean means dirty, or ritually impure. Unclean water, unclean hands, ritually unclean.
The plain sense is dirty and is slightly formal — unclean water is what a report says where a person says dirty. The stronger sense is religious: many traditions divide foods and states into clean and unclean, and the categories are about ritual rather than hygiene, which is why they do not map onto what is actually safe to eat. Note uncleanliness; and unclean spirit, which is a biblical phrase.
1adjectivedirtyDirty.
“The inspectors closed the restaurant because the kitchen was unclean.”
2adjectiveuncleanRitually impure.
“In some religions, pork is considered unclean and is never eaten.”
unclean is a quality of impurity.
Old English unclaene is un- plus clean, and the religious sense is as old as the plain one in English, arriving with the Bible translations.
Old English
unclǣne
Middle English
unclene
English, today
unclean
From Middle English unclene, from Old English unclǣne, equivalent to un- + clean.
Etymology from Wiktionary, CC BY-SA