The form in English was altered 16c. by influence of base (adj.), making the word an exception to the rule that Old French verbs with stem -iss- enter English as -ish (comprehension might have played a role; earlier forms of abase often are identical with those of abash). Literal sense of "lower, depress" (late 15c.) is archaic or obsolete. Related: Abased; abasing.
late 14c., obeisaunce, "act or fact of obeying, submissiveness, quality of being compliant or dutiful; respectful submission, homage," from Old French obeissance "obedience, service, feudal duty" (13c.), from obeissant, present participle of obeir "to obey," from Latin oboedire "to obey" (see obey). The sense in English altered late 14c. to "bending or prostration of the body as a gesture of submission or respect, a bow or curtsy; deferential deportment; an act of reverence or deference" by influence of abase. Related: Obeisant.
"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."
Originally, to put to confusion from any strong emotion, whether of fear, of wonder, shame, or admiration, but restricted in modern times to effect of shame. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]
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 opening the lips. Middle English Compendium also compares Old French abaissier "bow, diminish, lower oneself" (source of abase). Related: Abashed; abashing. Bashful is a 16c. derivative.
1560s, "lower in position, rank, or dignity, impair morally," from de- "down" + base (adj.) "low," on analogy of abase (or, alternatively, from obsolete verb base "to abuse"). From 1590s as "lower in quality or value" (of currency, etc.), "degrade, adulterate." Related: Debased; debasing; debasement.
"to cause to be or appear lower or more humble; depress, especially to abase in estimation; subject to shame or disgrace; mortify," 1530s, a back-formation from humiliation or else from Late Latin humiliatus, past participle of humiliare "to humble," from humilis "lowly, humble," literally "on the ground," from humus "earth" (from PIE root *dhghem- "earth"). Earlier was humily "humble oneself" (mid-15c.), from Old French humilier. Related: Humiliated.