late 14c., "reduce in rank, etc.," from Old French abaissier "diminish, make lower in value or status; lower oneself" (12c.), literally "bend, lean down," from Vulgar Latin *ad bassiare "bring lower," from ad "to, toward" (see ad-) + Late Latin bassus "low, short" (see base (adj.)).

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.

"humiliation proceeding from guilt, shame, or consciousness of unworthiness; degradation of oneself by one's own act," 1650s; see self- + abasement. 

common suffix of Latin origin forming nouns, originally from French and representing Latin -mentum, which was added to verb stems to make nouns indicating the result or product of the action of the verb or the means or instrument of the action. In Vulgar Latin and Old French it came to be used as a formative in nouns of action. French inserts an -e- between the verbal root and the suffix (as in commenc-e-ment from commenc-er; with verbs in ir, -i- is inserted instead (as in sent-i-ment from sentir).

Used with English verb stems from 16c. (for example amazement, betterment, merriment, the last of which also illustrates the habit of turning -y to -i- before this suffix).

The stems to which -ment is normally appended are those of verbs; freaks like oddment & funniment should not be made a precedent of; they are themselves due to misconception of merriment, which is not from the adjective, but from an obsolete verb merry to rejoice. [Fowler]