overindulgence n.

word-forming element meaning variously "above; highest; across; higher in power or authority; too much; above normal; outer; beyond in time, too long," from Old English ofer (from PIE root *uper "over"). Over and its Germanic relations were widely used as prefixes, and sometimes could be used with negative force. This is rare in Modern English, but compare Gothic ufarmunnon "to forget," ufar-swaran "to swear falsely;" Old English ofercræft "fraud."

In some of its uses, moreover, over is a movable element, which can be prefixed at will to almost any verb or adjective of suitable sense, as freely as an adjective can be placed before a substantive or an adverb before an adjective. [OED]

Among the old words not now existing are Old English oferlufu (Middle English oferlufe), literally "over-love," hence "excessive or immoderate love." Over- in Middle English also could carry a sense of "too little, below normal," as in over-lyght "of too little weight" (c. 1400), overlitel "too small" (mid-14c.), oversmall (mid-13c.), overshort, etc.

mid-14c., in the Church sense, "a freeing from temporal punishment for sin, remission from punishment for sin that remains due after absolution," from Old French indulgence or directly from Latin indulgentia "complaisance, a yielding; fondness, tenderness, affection; remission," from indulgentem (nominative indulgens) "indulgent, kind, tender, fond," present participle of indulgere "be kind; yield, concede, be complaisant; give oneself up to, be addicted," a word of uncertain origin. It is evidently a compound, and the second element appears to be from PIE root *dlegh- "to engage oneself, be or become fixed." The first element could be in- "in" for a sense of "let someone be engaged" in something, or in- "not" for a total sense of "not be hard toward" someone.

Sense of "leniency, forbearance of restraint or control of another, gratification of desire or humor" is attested from late 14c. That of "yielding to one's inclinations" (technically self-indulgence) in English is from 1630s. In British history, Indulgence also refers to grants of certain liberties to Nonconformists under Charles II and James II, as special favors rather than legal rights. The sale of indulgences in the original Church sense was done at times merely to raise money and was widely considered corrupt; the one in 1517 helped to spark the Protestant revolt in Germany.