This chapter explains the evolution of ‘hard-word’ dictionaries in English lexicography during the seventeenth century. Through a discussion of how lexicography and natural philosophy were emerging and how scientific methods were being applied to language, it traces the development of dictionaries and what kinds of information went into them. We see that the first monolingual dictionaries were lists of words; subsequently, those lists included short definitions of terms borrowed from other languages. Soon, dictionaries provided definitions of technical terms and jargon – the ‘hard words’ – and finally, dictionaries developed into encyclopedic reference works that added information on the history and development of the language and practical uses of a dictionary for words in general use. Last, the chapter details a famous case of lexicographical plagiarism to present how the concept of authorship has changed over time.