The month before last (two months ago). Commonly used in both speech and writing.
先々月、家族で温泉に行きました。
We went to a hot spring with the family two months ago.
先々月の売上は予想より高かった。
The sales for the month before last were higher than expected.
彼は先々月付けで海外支店に転勤した。
He was transferred to an overseas branch as of the month before last.