While editing my 1920s era manuscript, It Takes Moxie, I discovered there's a fine line between slang giving a story flavor and pulling a reader out of the flow of the story. I was asked to replace many of my 'fella's with the word 'guy'. I try to keep my dialogue as true to the era as possible, taking out 'fella' made me wince -– it would be like replacing 'gentleman' with 'guy' in a regency. But, I also understood my editor's point of how too many fellas are distracting (while reading, not in real life ;) ). Guy is such a common word today that it becomes somewhat invisible to a reader, fella doesn't. So, I cut my use of that word drastically.
But, that doesn't mean we have to get rid of all our slang. I also referred to someone as being 'a modern'. The word modern was used like a noun in the Jazz Age, meaning someone who was free-thinking and up to date on the trends. I fell in love with the term when someone referred to Joan Crawford as being a modern in the old silent movie, Our Dancing Daughters. I think using somewhat obscure terms throughout our books gives readers a little peek into the past. When you add music and clothing from the era, you can transport your readers to a time otherwise denied them. What a wonderful gift –- use it wisely.