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English word rat bastard (referring to a despicable person) comes from the combination of English rat and bastard. The exact origin of the expression is uncertain
bastard (English)
(obsolete) To bastardize. (UK, politics, pejorative) A Eurosceptic Conservative MP, especially in the government of John Major.. (informal) A child who does not know his or her father.. (informal) Something extremely difficult or unpleasant to deal with.. (often preceded by 'poor') A person deserving of pity.. (often, humorous) A man, a fellow, a male friend.. (vulgar, referring to a man) A [...]
rat bastard (English)
(slang, pejorative) A thoroughly despicable and untrustworthy person; a scoundrel.
rat
A rodent that resembles a large mouse.