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"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]