UrbanDictionary.com defines a coke nail as “one significantly longer fingernail on one or both hand(s), most often the pinkie nail, which is used for scooping up powder cocaine and snorting it. This is known as doing a bump.
Using your nails is faster and more handy (excuse the pun) than using a tool for this, and for a serious painter like Bob Ross there is no question that he used this old trick too.