to pass something off = (1) to make people believe that something is other than what it truly is, (2) to accept, understand something without further investigation or research.

(1a) I was able to pass the counterfeit bill as legitimate tender.  (People thought the money was legit, but it wasn´t.)

(1b) I passed off the brass ring as an 18 karat gold ring.

(2a) He was rude to me, but I passed it off as a bad mood related to his illness.

(2b) She seemed very conceited, but I passed it off as a bad case of nerves.

(2c) I passed off his tardiness as lack of interest in the job.  I learned later that he had been in an accident on his way to the office.

No sentido (2), o significado e parecido a "aceitar" ou "achar".