先せん々せん週しゅう

せ

ん

せ

ん

しゅ

う

Meanings

1

the week before last

Refers to the week immediately preceding last week. Can be used as a noun or adverbially.

その映えい画がは先々せんせん週しゅう見みました。

I saw that movie the week before last.

再さ来らい週しゅうN4
先々週 means 'the week before last', while 再来週 means 'the week after next' — they mirror each other on opposite sides of the current week.

先せん週しゅうN5
先週 means 'last week' (the week right before this one), while 先々週 means 'the week before last', one week further back — learners often mix them up because of the similar spelling.

先せん々せん週しゅう
Standard kanji spelling using the iteration mark 々.
先せん先せん週しゅう
Alternative spelling without the iteration mark; less common but still used.

Compound of 先 (previous) reduplicated with the iteration mark 々, plus 週 (week). The reduplication shifts the time reference one step further back.