I have a short black line under my left thumbnail. it looks like a splinter, but it way too far back in the nail. what could it be?

Probably benign: This is most likely the result of trauma to the nail bed. You have has a bit of bleed there and you see a hemorrhage. There are some serious causes of pigmented lesion under the nails such as melanoma. If the lesion is growing out of the nail over a 2-4 week time and not growing in size or changing character or color, then observe, otherwise, see your pcp or dermatologist and get checked out.

Black line: Black stripes down 1 nail may be subungual hematoma. Red or brown streaks may be splinter hemorrhages. Brown/black lines from nail plate to tip are often seen in people with dark skin & are normal. Can occasionally be due to illnesses.