With even stronger reason; for even better reason; used in logic to indicate that if something is true, something else must be even more certainly true.
From Latin 'a fortiori,' literally 'from [the] stronger [reason].' This Latin phrase was adopted directly into English legal and philosophical terminology.
Lawyers and philosophers love this phrase because it's a logical mic-drop: if X is true, then a fortiori (obviously!) Y is even more true.
Complete word intelligence in one call. Free tier — 50 lookups/day.