Full of; having given qualities.
Adjectives formed in this ending can mean ‘full of’: beautiful, colourful, graceful, painful, powerful, youthful. In other cases it means ‘having the qualities of’: careful, helpful, lawful, restful, spiteful, thoughtful, truthful, useful. Some, taken from English verbs, have the senses ‘apt to, able to, accustomed to’: boastful, forgetful, mournful, wakeful, watchful. Over time some have become figurative: grateful (formerly ‘agreeable, acceptable’, from Latin gratus, pleasing), now means ‘feeling or showing an appreciation of kindness’; hateful (once ‘full of hate', now ‘generating hate’), joyful (originally ‘full of joy’, rather than the more modern ‘causing or evoking joy’).
The ending generates nouns indicating an amount needed to fill a given container or holder: armful, basketful, cupful, handful, hatful, spadeful. A few examples of this type are figurative: earful, literally an ear-filling amount, but usually a severe reprimand; eyeful, a good look, but often describing somebody striking or attractive; and skinful, enough alcoholic drink to make one drunk.