2、古英语前缀a-, 来自PIE*uz, 向上，向外，词源同out. go, 走，离开。
2、ago:  Historically, ago is the past participle of a verb. Its earlier, Middle English, form – agone – reveals its origins more clearly. It comes from the Old English verb āgān ‘pass away’, which was formed from gān ‘go’ and the prefix ā- ‘away, out’. At first it was used before expressions of time (‘For it was ago five year that he was last there’, Guy of Warwick 1314), but this was soon superseded by the now current postnominal use.=> go
4、early 14c., shortened form of Old English agan, agone"departed, passed away,"past participle of an obsolete verb ago"to go forth,"formed from a-"away"(perhaps here used as an intensive prefix) + gan"go"(see go (v.)). Agone remains a dialectal variant.