1610s, "allowable," from French admissible, from past participle stem of Latin admittere "allow to enter, admit, give entrance," from ad "to" (see ad-) + mittere "let go, send" (see mission). Meaning "capable of being allowed entrance" is from 1775; specific sense of "capable of being used in a legal decision or judicial investigation" is recorded from 1849.
In Middle English, also "sex between husband and wife for recreational purposes; idolatry, perversion, heresy." As a crime, formerly classified as single adultery (with an unmarried person) and double adultery (with a married person). The Old English word was æwbryce "breach of law(ful marriage)" (similar formation in German Ehebruch). In translations of the 7th Commandment it is understood to mean "lewdness or unchastity" of any kind, in act or thought.
c. 1500, "act of adulterating; state of being debased by mixture with something else," generally of inferior quality, from Latin adulterationem (nominative adulteratio) "an adulteration, sophistication," noun of action from past-participle stem of adulterare "corrupt, falsify; debauch; commit adultery," from ad "to" (see ad-) + alterare "to alter" (see alter), though Watkins explains it as ad alterum "(approaching) another (unlawfully)." Meaning "a result of adulterating" is from 1650s.