Does fill mean full?

Fill and full are related but not identical in meaning. Fill is primarily a verb meaning to make something full, to cause something to become full by adding contents, such as fill a glass with water. Full is primarily an adjective meaning containing as much as possible, not lacking space, or having reached capacity, such as a full glass. So fill is the action that results in the state of being full. For example, you fill a bucket, and then the bucket is full. They share same Old English root, fyllan and full, both related to fullness. In some informal uses, fill can be used as noun meaning enough, as in eat your fill, which is close to full. But grammatically they function differently, fill is usually verb, full is adjective or adverb. You cannot generally substitute one for the other, you would not say the glass fill, you say the glass is full. So fill does not directly mean full, it means to make full, to cause fullness. The result of filling is being full.