Etymology
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legal (adj.)

mid-15c. "of or pertaining to the law," from Old French légal "legal" (14c.) or directly from Latin legalis "pertaining to the law," from lex (genitive legis) "an enactment; a precept, regulation, principle, rule; formal proposition for a law, motion, bill; a contract, arrangement, contrivance." This probably is related to legere "to gather," from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Perhaps the noun is from the verb on the notion of "a collection of rules," but de Vaan seems to imply that the evolution is the reverse:

The verb legare and its compounds all have a meaning which involves a 'task, assignment,' and can therefore be interpreted as derivatives of lex 'law.' The [Proto-Italic] root noun *leg- 'law' can be interpreted as a 'collection' of rules. Whether the root noun existed already in PIE is uncertain for lack of precise cognates.

Sense of "permitted by law" is from 1640s. Related: Legally. Not etymologically related to law (n.), q.v. The usual Old French form was leial, loial (see leal, loyal). Legal tender "money which the creditor is bound by law to accept" is from 1740 (see tender (n.2)). A legal holiday (1867) is one established by statute or proclamation and during which government business is usually suspended.

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medico-legal (adj.)

"of or relating to law and medicine," by 1824, from medico- + legal.

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legalese (n.)
"the language of legal documents," 1914, from legal + language name ending -ese.
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legalize (v.)
1716, from legal + -ize. Related: Legalized; legalizing.
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paralegal (n.)

"one trained in subsidiary legal matters," 1972, from para- (1) + legal assistant.

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legalistic (adj.)
1843, with -ic + legalist "one who advocates strict adherence to the law," especially in theology (1640s); see legal + -ist. Legalism in theology is attested from 1838.
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legality (n.)

mid-15c., "law-abiding behavior or character," from Medieval Latin legalitatem (nominative legalitas), from Latin legalis "pertaining to the law" (see legal).

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legicide (n.)
"a destroyer of laws," 1680s, from Latin legis, genitive of lex "law" (see legal (adj.)) + -cide "killer."
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tender (n.2)
"formal offer for acceptance," 1540s, from tender (v.). Specific sense of "money offered as payment" is from 1740, in legal tender "currency which by law must be accepted from a debtor" (see legal).
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in custodia legis (adv.)
legal Latin, "in the custody of the law," from ablative of custodia "a guarding, watching, keeping" (see custody) + legis, genitive of lex "law" (see legal (adj.)).
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