Thick-billed Flowerpecker is about 10 cm long and has a dark stout beak and short tail. They are dark grey brown above and dull greyish with diffuse streaking on light buffy underparts. The rump is slightly more olive in the nominate race. The bill is dark, somewhat stout and heavy and the iris is reddish. The sexes are not distinguishable in the field and the juvenile has a paler base to the mandible and less streaks on the underside. There are whitish spots at the tip of the tail feathers.
They feed predominantly on fruits and are active birds that are mainly seen in the tops of trees in forests.