Alternate spelling of "oof", which is an onomatopoeia (word that sounds like the
noise it describes or refers to, in this
case the sound a person makes upon impact with an unexpected
force that knocks the breathe out of them).
It is more of a 'sound effect' than an actual word, used to indicate that someone feels the impact of a powerful or meaningful statement someone made, or sympathizes with the crushing
blow another person experienced. In this regard, it's a synonym for "ouch".
This alternate spelling
may be used as a stylistic preference, or to avoid the confusion of this now somewhat uncommonly used word being mistaken as a typo of "of" or "off".