From The Collaborative International Dictionary of English v.0.48:
Ago \A*go"\, a. & adv. [OE. ago, agon, p. p. of agon to go away,
pass by, AS. [=a]g[=a]n to pass away; [=a]- (cf. Goth. us-,
Ger. er-, orig. meaning out) + g[=a]n to go. See Go.]
Past; gone by; since; as, ten years ago; gone long ago.
[1913 Webster]