This is the 706th most frequent Japanese word.
だって
"だって" means "because," "but," or "after all," and can also indicate quoting or emphasis.
In this example, 'だって' denotes a reason or cause for the following statement, translated as 'because'.
だって疲れているんだもん。
Because I'm tired.
Here, 'だって' emphasizes inclusion, meaning 'even' or addressing someone mentioned as involved.
彼女、だってこの計画に賛成してるよ。
Even she agrees with this plan.
In this usage, 'だって' conveys a sense of surprise or introducing something, similar to 'well' or 'guess what'.