Iff
/ɪf/
Definition 1 of 1
Pro Tip 1/3Mathematical Precision
Used for precise conditions in math and logic, where 'if' alone is insufficient. 
A square is a rectangle iff all angles are 90 degrees.
Pro Tip 2/3Not Common Speech
Rare in everyday language; primarily found in technical or academic contexts. 
He's not using 'iff' because it's a casual conversation, not a math class.
Pro Tip 3/3Double F Not Mistake
'iff' isn't a typo for 'if'; the double 'f' is intentional to mean a stronger condition. 
I'll agree iff you provide concrete evidence.