A sexual reference to the common act of 'fixing, holding and/or playing with a woman's hair,' but only while she is giving a man a blowjob.
Other common forms of "fixing that hair" include:
'fixin her hair' and 'fixing the way you wear that hair,' which is used to directly insult the person being spoken to.
Example - Man 1: Hey, did you and Jane end up messing around last night?
Man 2: Oh! You know i was fixing that hair.