One of many stock Homeric epithets, apparently used as mnemonic and prosodic padding, to help with recital from memory and to meet the exigencies of the dactylic hexameter. I like it, anyhow.
See also the bestellar reviews, complete with rich quote-mosaics, of Adam Nicolson’s magnificent Why Homer Matters, and Christopher Logue’s War Music, a muscular rendition of several books of the Iliad.
‘Then in answer again spoke brilliant swift-footed Achilleus.’
Source: Homer, The Iliad of Homer, trans. Richmond Lattimore (Chicago: Chicago UP, 1961 (1951)), book I, p. 62