Douglas Harper's Etymology Dictionary
Middle French, "shame on him who thinks evil of it;" proverbial expression recorded from c.1300, used as motto of the Order of the Garter.
Wikipedia
Honi soit qui mal y pense (UK: or US: ) is an Anglo-Norman maxim which rather means, "Shame on whosoever would think badly of it," or "May he be shamed who thinks badly of it".
Its literal translation from Old French is "Shame be to him who thinks evil of it." It is sometimes re-interpreted as "Evil (or shame) be to him that evil thinks." In contemporaneous French usage, it is usually used ironically, to insinuate the presence of hidden agendas or conflicts of interest.
The saying's most famous use is as the motto of the British chivalric Order of the Garter. It is also inscribed at the end of the manuscript of Sir Gawain and the Green Knight, but that appears to have been a later addition.
Usage examples of "honi soit qui mal y pense".
On Sunday morning there were reports of a naked man picketing on Lakeside Avenue with a large sign saying: HONI SOIT QUI MAL Y PENSE.
The description of all the Hermione mutineers had been circulated to every naval station: 'George Norris, gunner's mate, aged 28 years, five feet eight inches, sallow complexion, long black hair, slender build, has lost the use of the upper joint to his forefinger of the right hand, tattooed with a star under his left breast and a garter round his right leg with the motto Honi soit qui mal y pense.