mos

Mos, moris is third declension masculine, a consonant stem, genitive plural morum. Singular and plural do noticeably different jobs. In the singular it is one custom or a person's own way of doing things; in the plural, mores is character and morals, the whole habitual conduct of a person or a people, so boni mores is not "good customs" but something nearer "good character".

The ablative more with an adjective is standard for manner: more Romano, "in the Roman way", more maiorum, "as our ancestors did". And mos maiorum is a technical term as much as a phrase: the ancestral way functioned at Rome as unwritten constitutional authority, standing beside lex rather than beneath it. Family: moralis, coined by Cicero to render Greek ethikos, morosus, mos gerere, "to humour". English moral, morale, morose, and mores borrowed whole by sociology.