Yiffy generally means feeling horney, or to look sexy.
Oddly enough Yiff is a large picture in the furry subculter. the majority of furs are rather permisques & open about sex. along simular lines about 85-95% of furs are either gay or bi (leaving myself in the minority for once) this openness is also another reason why most atrwork is erotic & most furs are past puberty.