Time, as you know, zips along. One generation changes another. Habits, values of live, people's customs change, and language they speak changes along with them. Language reflects trendiness of epoch, and it is its mirror. Things, receding into the past, are concidered history. Even certain words called historicisms exist. They grow out of use and are not mentioned in the speech any more.
A different matter concerns archaisms. These are such obsolete word, which were replaced by synonyms in modern language. Sometimes they keep their stem in the root of a word, and sometimes are completely transformed into another. We can meet their primary meaning only when we need to reconstruct or to lay emphasis on the colouring of certain historical era (literary works, theatrical performances, fictional or documentary cinematograph etc).
Examples of archeisms: токмо – только /only/, посему – поэтому /that's why/, глас – голос /voice/, изволить – желать /to wish/, ежели – если /if/, чело – лоб /forehead/, откушать – поесть /to have a meal/, потчевать – кормить /to feed/, уста – губы /lips/, вечор – вечер /evening.