Meaning of AGO in English

AGO

■ adverb before the present (used with a measurement of time).

Origin

ME ~ , ~ne , past participle of the obs. verb ~ 'pass', used to express passage of time.

Usage

When ~ is followed by a clause, the clause should be introduced by that rather than since , e.g. it was sixty years ~ that I left this place (not it was sixty years ~ since I left this place ).

Concise Oxford English vocab.      Сжатый оксфордский словарь английского языка.