Either an adoption of the Gaelic topographic term ros ‘headland’ (compare Glen, Kyle) or a transferred use of the Scottish surname, which is borne by a large and ancient family whose members have played a major role in Scottish history. Although still very popular in Scotland, the name is now in widespread general use.
English and Scottish: patronymic from the personal name John. As an American family name, Johnson has absorbed patronymics and many other derivatives of this name in continental European languages. (For forms, see Hanks and Hodges 1988.)