YOU SAID:

I solemnly swear that I'm up to no good

INTO JAPANESE

私は厳しく自分がダメだと誓います

BACK INTO ENGLISH

I strictly swear I am no good

INTO JAPANESE

私は絶対にダメだと誓います

BACK INTO ENGLISH

I swear I absolutely can't

INTO JAPANESE

私は絶対にできないと誓います

BACK INTO ENGLISH

I swear I can't

INTO JAPANESE

できないと誓う

BACK INTO ENGLISH

I swear I can't