I must be missing something here. https://carefulwords.com/fast gives back 349 results, starting with "Barmecidal feast, Encratism, Friday, Lenten diet, Lenten entertainment, Lenten fare, Pythagoreanism, Pythagorism, Rechabitism, Sabbath, Shakerism, Spartan fare, Stoicism, Sunday" and also containing "ecclesiastical calendar, extravagant, firm as Gibraltar, hell for leather, hell-bent, promiscuous" and many many others which, I believe, are nowhere near being synonimous with "fast".