From Longman Dictionary of Contemporary Englishbe minding your own businessbe minding your own businessto be doing something ordinary on your own when something unexpected happens to you My father was just driving along, minding his own business, when suddenly a brick came through the window. → mind
Examples from the Corpus
be minding your own business• It's a bit disconcerting to be minding your own business.• I was minding my own business, sleeping, when I heard something. 