Run of the mill means the usual, not out of the ordinary, unspectacular. The idiom run of the mill comes from a time when machinery began to be used in factories and mills, items that were once custom-made by hand were now cranked out for consumption by the masses. The idiom run of the mill was first used in the early 1900s in reference to a weaving mill, which put out unremarkable clothes for consumption by the masses. Run of the mill may be hyphenated as run-of-the-mill, but like many hyphenated words, the hyphens are quickly disappearing.
Examples
Journalist Kevin McAnena, who was in the foyer of the hotel as the gunmen entered, described how a “run of the mill” boxing weigh-in ended up with him “staring down the barrel of gun”. (The Independent)
The villains are run of the mill types- a businessman (Narendra Jha) who lives in an Antilla type of a building. (The Navhind Times)
Learning how Israel does that, as well as how it defends individuals, companies, and governments from more run of the mill cyber-threats like hackers who steal credit card information or breach databases, brought over 3,000 people to Cybertech 2016, an annual event organized by IsraelDefense magazine and academic, corporate, and government sponsors. (The Times of Israel)
Your run of the mill in-house agencies can’t attract and retain the same caliber of creative talent that Apple does. (Forbes Magazine)
If you’re a recent traveler who does have any one of the four symptoms, you should seek a doctor’s help regardless of whether or not you think it’s Zika virus, as the disease shares symptoms with other tropical or mosquito-borne diseases like malaria and Chikungunya virus and more run-of-the-mill but serious things like measles, rubella and enterovirus. (The Huffington Post)