"substances used in medicine," 1690s, Latin, literally "medical matter."
"in the matter of, in the (legal) case of," c. 1600, probably from Duns Scotus; Latin, from re, ablative of res "property, goods; matter, thing, affair," from Proto-Italic *re-, from PIE *reh-i- "wealth, goods" (source also of Sanskrit rayi- "property, goods," Avestan raii-i- "wealth").
Latin word once used in various phrases in English, often in legal language, where it means "the condition of something, the matter in hand or point at issue;" literally "thing" (see re). For example res ipsa loquitur "the thing speaks for itself;" res judicata "a point decided by competent authority."
1832, Latin, literally "body of the offense;" not "the murder victim's body," but the basic elements that make up a crime, which in the case of a murder includes the body of the victim. For first element, see corpus. With delictum "a fault, offense, crime, transgression," etymologically "a falling short" of the standard of law, neuter singular of past participle of delinquere "to fail; be wanting, fall short; offend" (see delinquent).
Thus, a man who is proved to have clandestinely buried a dead body, no matter how suspicious the circumstances, cannot thereby be convicted of murder, without proof of the corpus delicti--that is, the fact that death was feloniously produced by him. [Century Dictionary]