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early 13c., from Old French blasfemie "blasphemy," from Late Latin blasphemia, from Greek blasphemia "a speaking ill, impious speech, slander," from blasphemein "to speak evil of." Second element is pheme "utterance" (see fame); first element uncertain, perhaps related to blaptikos "hurtful," though blax "slack (in body and mind), stupid" also has been suggested.
In the sense of speaking evil of God this word is found in Ps. 74:18; Isa. 52:5; Rom. 2:24; Rev. 13:1, 6; 16:9, 11, 21. It denotes also any kind of calumny, or evil-speaking, or abuse (1 Kings 21:10; Acts 13:45; 18:6, etc.). Our Lord was accused of blasphemy when he claimed to be the Son of God (Matt. 26:65; comp. Matt. 9:3; Mark 2:7). They who deny his Messiahship blaspheme Jesus (Luke 22:65; John 10:36). Blasphemy against the Holy Ghost (Matt. 12:31, 32; Mark 3:28, 29; Luke 12:10) is regarded by some as a continued and obstinate rejection of the gospel, and hence is an unpardonable sin, simply because as long as a sinner remains in unbelief he voluntarily excludes himself from pardon. Others regard the expression as designating the sin of attributing to the power of Satan those miracles which Christ performed, or generally those works which are the result of the Spirit's agency.
irreverence toward a deity or deities and, by extension, the use of profanity.