Fluffing is a term used with reference to Cuckolding.
Brief Sentence About Cuckolding
The cuckold is the partner that often has a small dick and encourages his wife to have a lover that can satisfy her more fully. The wife/partner is called the Hotwife. The lover is called the Bull.
So What is Fluffing?
Fluffing is (usually) an advanced portion of cuckolding when the cuckold gets the Bull ready for the Hotwife by stroking him (the Bull) to erection or sucking his cock to get him primed for the pleasure of the pussy. This is made all the more entertaining by the cuckold being straight and humiliated to perk up the Bull’s dick.