How Well Do You Know English Slang?
"modest, bashful," 1550s, folk etymology alteration of shamefast, 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]