"modest, bashful," 1550s, a folk-etymology alteration of shamefast, "modest, humble, virtuous," also "ashamed of one's behavior," from Old English scamfæst "bashful," literally "restrained by shame," or else "firm in modesty," from shame (n.) + -fæst, adjectival suffix (see fast (adj.)). Related: Shamefacedly; shamefacedness.
shamefaced, -fast. It is true that the second is the original form, that -faced is due to a mistake, & that the notion attached to the word is necessarily affected in some slight degree by the change. But those who, in the flush of this discovery, would revert to -fast in ordinary use are rightly rewarded with the name of pedants .... [Fowler]
<a href="https://www.etymonline.com/word/shamefast">Etymology of shamefast by etymonline</a>
Harper, D. (n.d.). Etymology of shamefast. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/shamefast