How do you know what a word means?
We don't always know! Lexicographers generally have wide vocabularies and fairly good general knowledge, but none of us can hope to be familiar with all the words that we deal with in the course of our work. We discover the meaning of an unfamiliar word by looking at our databases of language and by consulting reference works.
Dictionaries are descriptive, not prescriptive. In other words, when we're defining a word we combine our understanding of how it is used in the language with an analysis of resources such as the Oxford English Corpus. This should result in a definition of how a word is actually used, rather than how we think it should be used.
For scientific and technical terms we make use of our well-stocked library or the Internet, or we consult experts with specialist knowledge.
See other FAQs about dictionaries.