Old English forgiefnes, forgifennys "pardon, forgiveness, indulgence," from past participle of forgifan (see forgive) + -ness. Contracted from *forgiven-ness. Middle English also had forgift (early 14c.).
Old English forgiefan "give, grant, allow; remit (a debt), pardon (an offense)," also "give up" and "give in marriage" (past tense forgeaf, past participle forgifen); from for-, here probably "completely," + giefan "to give" (from PIE root *ghabh- "to give or receive").
The sense of "to give up desire or power to punish" (late Old English) is from use of such a compound as a Germanic loan-translation of Vulgar Latin *perdonare (Old Saxon fargeban, Dutch vergeven, German vergeben "to forgive," Gothic fragiban "to grant;" and see pardon (n.)). Related: Forgave; forgiven; forgiving.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
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Harper, D. (n.d.). Etymology of forgiveness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/forgiveness