Dissemble means to conceal your thought or feeling. I figured out my definition because I know that dis mean "not" and semble means to seem. So if you attach the words together it is "not seem." This proves that the word dissemble means to conceal or hide your thoughts.
Spark notes online has a really good summary I used and it helped me without me even having to read the book
Cognates are words from different languages that look similar and mean the same thing.