giants and the gods was hit by arrows in each eye, from the bows of Apollo and Heracles. Most of the giants have sinister names, and this is no exception, since it means 'he who leaps on', from epi, 'on' and iallo, 'to put'.
This seems to indicate a personified nightmare. There is, however, a later story that tells how he and his brother Otus ('pusher') piled up mountains, this being the 'putting on'. Of course, one who leaps on someone could perhaps be nothing more than some kind of personification of the wind.