The word "-ful" is a suffix. It is used in the English language and is taken from the Middle English having been derived from the Old English word "full". The suffix is used to refer to something as being full of something, characterized by or given, able and tending to be something. It is used in a quantitative sense to refer to something as being filling. A child who is of a pleasant temperament can be said to be playful, s person who is generally ready to help others will be termed helpful, something that does not contain in ones arms can be said to be armful, that which will be of some use is useful, when some thing is provided for in abundance it is written as bountiful etc.
