what are ordinary and necessary expenses? common and accepted in the general industry or type of activity in which the taxpayer is engaged. an expense that is appropriate and helpful in furthering the taxpayer's business or income-producing activity. common and accepted in the general industry or type of activity in which the taxpayer is engaged. an expense that is mandated by the irs. expenses that are common to most businesses. an expense that is appropriate and helpful in furthering the taxpayer's business or income-producing activity. expenses that are common to most businesses. an expense that is mandated by the irs.