Literally: eight – face – six – elbow
Alternately: Active in a variety of areas, usually with incredible energy and results. One person doing the work of several. As productive as if having eight heads and six arms.
Notes: One variant uses three (三, san) faces instead of eight. The 三面 version seems to be the original, based on depictions of Asura, but it’s used less frequently in contemporary Japanese. While the two are more or less interchangeable, if you contrast them, the 八面 version can imply a broader (or unlimited) range of activity and accomplishment – cf. the use of the number eight to mean “all directions.”