先せん々せん月げつ

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Meanings

1

month before last

Refers to the month two months before the current month. Often used with time expressions like の or as an adverb.

先々せんせん月げつの請せい求きゅう書しょが見みつからない。

I can't find the invoice from the month before last.

先々月せんせんげつに〈動詞どうし〉〉to do something during the month before last
先々せんせん月げつに引ひっ越こしをしました。
I moved two months ago.
先々月せんせんげつから〈動詞どうし〉〉to have been doing something since the month before last
先々せんせん月げつから日本にほん語ごを勉べん強きょうしています。
I've been studying Japanese since the month before last.