A grullet is essentially, a greasy mullet. It may repulse, and is therefore to be used in a disgusted and/or frightened way.
uugh that guy's mullet is so greasy...I think that's what you call a grullet!
Holy s**t! I thought there was a monster on that guy's head...phew, it's only a grullet.
Not even white spirit could stop that grullet now, soon, that guy will BE a grullet!
Quite simply, a 'greasy mullet'. This is the state hair gets in when you don't wash it for a couple of days, stand in smoky pubs, eat a few too many kebabs and finally dip your head in a last nights chip fat. A Gomper will often be found with hair such as this.
Once again, after a few nights worth of burning the candle at both ends, James's hair resembled a grullet.