The spelling "laff" is a simplified, informal spelling that's often used in informal writing or online communication. It's like how "u" can be used instead of "you".
The reason for the difference between the spelling and pronunciation is historical:
* Old English: The word "laugh" originally came from the Old English word "hlæhhan," which was pronounced with a different sound than the modern "f".
* Great Vowel Shift: Over time, the pronunciation of English words changed dramatically due to the Great Vowel Shift. This shift affected the vowel sounds in words, and the "h" sound in "hlæhhan" eventually disappeared, leaving the "f" sound we hear today.
So, although the spelling might suggest "laff", the correct pronunciation is still with a clear "f" sound.