What could it mean if you have dark reddish purple horizontal lines come under all your fingernails?

See a physician: I think the safest thing to do is see a doc. This could be nothing, just small trauma to your fingers, to certain deficiencies, to some pretty severe cardiac issues. Have you been extremely sick lately? Sometimes serious illness can cause this afterwards. Are you having any heart or breathing issues? If so see a doctor sooner. Without more history, i would say the safest thing is to see a dr.