Throughout Gawain and the Green Knight, one idea is emphasized over and over: the importance of following the rules. In Arthur’s court and during Gawain’s adventures, these guidelines are represented in the rules of appropriate behavior and conduct, the rules of religion, and the rules of fate. The phrase Honi Soit Qui Mal Y Pense, shame on him who thinks evil of it (OED), seems to be a warning to discourage those that would do evil by not following these guides.
To the Knights of the Round Table, purity, chivalry, and bravery were of the utmost importance. These were the characteristics by which knights defined themselves and for which they were praised. These rules of courtesy and manners define all the characters’ actions. Arthur, noble and well-mannered as he is, invites the fearsome Green Knight to dine with them and accepts his ridiculous challenge when the Green Knight questions his bravery; the Lord of the castle that Gawain stumbles upon unhesitatingly invites the stranger to celebrate the New Year with 
him; and Gawain kisses the Lord’s wife for fear of appearing uncourteous. Of all the knights, Gawain in particular is singled out as being especially virtuous – his shield depicts a pentangle, one of its five points representing his:
fraunchyse and felaghschyp forbe al thing,
His clannes and his cortaysye croked were never,
And pite, that passez alle poyntez: thyse pure five
Were harder happed on that hathel then on any other”
…generosity and love of fellow-man above all;
His purity and courtesy were never lacking,
And surpassing the others, compassion: these noble five
Were more deeply implanted in that man than any other
(Sir Gawain and the Green Knight, lines 652-655)
Another guide that exerts a powerful force on Gawain and the rules of society is that of religion. The narrator, as well as minor characters in the story, and even Gawain himself see God as watching over the brave knight. He rode across England “on Godez halve” in God’s name (692), would have been killed many times if he did not “Dryhhtyn…serve” trust in God (724), and, when facing his fate at the Green Chapel, Gawain reassures himself that “’To Goddez wylle I am ful bayn,/ And to hym I haf me tone.’” My life is in his hands,/ His will I shall obey (2158-2159). The will of God is in many ways synonymous to fate, and Gawain accepts his path unquestionably, certain that what happens is what is meant to be and courageous largely because of his strong faith.
The phrase Honi Soit Qui Mal Y Pense relates most closely to the moment when Gawain does not follow these rules. By not giving up the girdle the lady promises will keep him safe, Gawain reveals that he does not fully trust the system set up by the rules of courtesy, religion, and fate. Instead of following either the rules of the game or of good manners, he instead, understandably, gives in to his fear. His action then becomes one of deceit and cowardice. Almost immediately, Gawain’s puity and virtue is marred. Physically, his purity is blemished by the cut on his neck bestowed by the Green Knight. Mentally, his virtue is diminished by his recognition of his own moment of cowardice. He is confronted with his moment of weakness and decides to forever after wear the girdle, as a reminder and a way to “lehthe [his] hert” humble [his] heart (2438).
Through the story of Gawain, Honi Soit Qui Mal Y Pense becomes a saying for the values of the knights. They must devote themselves to the rules of chivalry and to follow, with utmost adherence, God’s plan for them, otherwise they risk every value that qualifies them for knighthood.
