Legal ends in -al, a standard Latin adjective suffix, and English took it in the mid-1400s from Old French légal or directly from Latin legalis, "pertaining to the law," from lex, "an enactment, a rule."
The interesting question is what lex itself comes from, and the scholars disagree in an instructive way. It is probably related to legere, "to gather" — the root behind collect, lecture and select — in which case a law is a collection of rules. But one recent authority argues the derivation runs the other way, with the verb's senses of task and assignment deriving from the noun. English also has the French-worn version of the same root in loyal and in leal, and the direct Latin borrowings legal, legislate and legitimate — the usual pattern of one root arriving twice at different levels of formality.
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. also from mid-15c.
legal came in through Latin, and its spelling follows that family's conventions.
How that family writes these sounds
| the abstract endings | -tion/-sion, -ity, -ous, -ate, -al, -ive, -arynation, gravity, various, create, legal, active |
What gives it away on the page
| -tion/-sion, -ity, -ous, -ate, -al, -ive, -ary | The standard Latin suffix set, usually stacked on a Latin root.nation, gravity, various, create, legal, active |