To rotate a point \( (x, y) \) 270 degrees counterclockwise about the origin, you can use the rotation transformation formula:
\[ (x', y') = (y, -x) \]
For the point \( (5, 8) \):
- \( x' = y = 8 \)
- \( y' = -x = -5 \)
So, rotating \( (5, 8) \) 270 degrees counterclockwise about the origin gives us the new point:
\[ (8, -5) \]
Thus, the answer is \( \boxed{(8, -5)} \).