To administer chloroform to someone in order to render them unconscious or to kill them painlessly.
From chloroform + -ize suffix meaning 'to treat with, affect with, or cause to become'. This verb emerged in the 1850s as chloroform became widely used in medical practice.
Unlike the instant knockout portrayed in movies, actually chloroformizing someone was a slow, uncertain process that required the chemist to gradually increase the dose while watching carefully for the right depth of unconsciousness—too little and the patient suffered, too much and they died.
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