YOU SAID:
I haven't seen something that obtuse in a long time
INTO JAPANESE
私は長い間鈍いものを見ていない
BACK INTO ENGLISH
I haven't seen anything dull for a long time.
INTO JAPANESE
私は長い間鈍いものを見ていません。
BACK INTO ENGLISH
I haven't seen anything dull for a long time.
That didn't even make that much sense in English.