a- -ing

Du vieil anglais an, qui est devenu la préposition on et du suffixe -ing.

a- -ing \ə ɪŋ\

1. (Archaïsme) Circonfixe pour former le participe présent d’un verbe.
   • Gather ye rosebuds while ye may,
     Old time is still a-flying:
     And this same flower that smiles to-day
     To-morrow will be dying.
     The glorious lamp of heaven, the sun,
     The higher he’s a-getting, … — (Robert Herrick, To the Virgins, to Make Much of Time, 1648)