A Fortiori Definition

ä fôrtē-ôrē, ā
adverb
For a still stronger reason; all the more.
American Heritage
All the more.
Webster's New World

With stronger or greater reason; as a corollary implied by a stronger claim.

Wiktionary
verb
By the stronger (reason). To draw an inference that when one proposition is true, then a second proposition must also be true, especially if the second is included in the first. For example, if a 19 year old is legally an adult, then a 20 year old is, too.
Webster's New World Law
adjective

With stronger or greater reason; as a corollary implied by a stronger claim.

Wiktionary

Origin of A Fortiori

  • Latin ā fortiōrī ā from fortiōrī ablative of fortior stronger

    From American Heritage Dictionary of the English Language, 5th Edition

  • From Latin ā (“from”) and fortiōrī, comparative of fortis (“strength”).

    From Wiktionary