Ingrown hairs are hairs that have curled around and grown back into your skin instead of rising up from it. Sometimes, dead skin can clog up a hair follicle. That forces the hair inside it to grow sideways under the skin, rather than upward and outward. Sometimes, cutting naturally curly hair too closely will result in the sharpened end of the hair piercing the skin, causing an ingrown hair. Ingrown hairs aren't serious. But they can be irritating and embarrassing.