why is there such an emphasis on the words honest and honestly throughout this scene act three scene three

These scenes underscore the lines between honesty and deception. These traits are at play on multiple levels with multiple characters. Cassio seems to be one of the only "honest" people in the play; he discloses his plans, his thoughts, and his concerns to Iago, omitting little or nothing. It is therefore ironic that he refers to Iago as "honest", since Iago is the complete opposite of everything Cassio is. By this point,"honest" has become tainted by its over-association with Iago; it has no simple meaning and is a watchword for shady dealings.