Animals that drink blood, like vampires, mosquitoes, or leeches; also slang for unethical people who take advantage of others.
'Blood' from Old English 'blod' plus 'sucker' from 'suck' (draw in). Originally literal for creatures, the metaphorical use for greedy or unethical people evolved by the 1800s.
Leeches are legitimate medical bloodsuckers—hospitals use them for reconstructive surgery because their anticoagulant saliva keeps blood flowing, which helps reattached fingers and ears survive!
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