legal (adj.)youdaoicibaDictYouDict[legal 词源字典]
mid-15c. "of or pertaining to the law," from Middle French légal or directly from Latin legalis "legal, pertaining to the law," from lex (genitive legis) "law," possibly related to legere "to gather," on notion of "a collection of rules" (see lecture (n.)).

Sense of "permitted by law" is from 1640s. Related: Legally. The Old French form was leial, loial (see leal, loyal). Legal tender is from 1740.[legal etymology, legal origin, 英语词源]
tender (v.)youdaoicibaDictYouDict
"to offer formally," 1540s, from Middle French tendre "to offer, hold forth" (11c.), from Latin tendere "to stretch, extend" (see tenet). The retention of the ending of the French infinitive is unusual (see render (v.) for another example). The noun meaning "formal offer for acceptance" is from 1540s; specific sense of "money that may be legally offered as payment" is from 1740; hence legal tender "currency."