abash (v.) Look up abash at Dictionary.com
"perplex or embarrass by suddenly exciting the conscience, discomfit, make ashamed," late 14c., earlier "lose one's composure, be upset" (early 14c.), from Old French esbaiss-, present stem of esbaer "lose one's composure, be startled, be stunned."

The first element is es "out" (from Latin ex-; see ex-). The second may be ba(y)er "to be open, gape" (if the notion is "gaping with astonishment"), possibly ultimately imitative of yawning. Middle English Dictionary also compares Old French abaissier "bow, diminish, lower oneself" (source of abase). Related: Abashed; abashing. Bashful is a 16c. derivative.