Stands for "Huge In All Formats". This acronym is used when a professional athlete makes a big play that would help owners of that player in fantasy leagues. The opposite of HIAF is DIAF which stands for Droppable in All Formats.
Announcer: Albert Pujols had 3 home runs, 5 hits, and 4 runs!
Fantasy Analyst: HIAF!
Lebron James: Just look at my stats bro.
Fan: HIAF! Keeper status!