先々週 (sensenshū) means “the week before last”

More examples

私は先々週から、毎朝歩いています。watashi wa sen sen shū ka ra mai asa aru i te i ma su

I have been walking every morning since the week before last.

Treats last week as the reference point and moves one full week farther into the past.

二週間前nishūkanmae: Indicates a point fourteen days before a stated or understood time.

先週より一週前の時期に。sen shū yo ri ichi shū mae no ji ki ni

先々週 (sensenshū) is formed by repeating 先 (sen, "before") before 週 (shū, "week"), extending 先週 (senshū) one week farther into the past.

In everyday English, two weeks ago is often more direct than the week before last, although both identify the same time.