A friend of mine told me English demonyms, words that identify people from a particular place (Roman, Japanese, Dutch etc.), largely depend upon the historical period in which the term originates. However, looking at the list on wikipedia (https://en.wikipedia.org/wiki/Demonym), it's difficult for me to see a specific historical pattern. Is there anyone here who knows anything about how history has influenced the use of English demonyms? Can we associate a particular demonym with a certain time in history?
Sorry for the vague question. Any insight would be appreciated ^^