"impious or profane speaking of God or sacred things," 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 phēmē "utterance" (from PIE root *bha- (2) "to speak, tell, say"); first element uncertain, perhaps related to blaptikos "hurtful," though blax "slack (in body and mind), stupid" also has been proposed; de Vaan suggests a connection with the root of Latin malus "bad, unpleasant" (from PIE root *mel- (3)). In Old Testament usage the word applied to a more specific crime, against the reverence for Jehovah as ruler of the Jews, comparable to treason.
Blasphemy cognizable by common law is described by Blackstone to be "denying the being or providence of God, contumelious reproaches of our Saviour Christ, profane scoffing at the Holy Scripture, or exposing it to contempt or ridicule"; by Kent as "maliciously reviling God or religion"; and by Chief Justice Lemuel Shaw as "speaking evil of the Deity with an impious purpose to derogate from the Divine Majesty and to alienate the minds of others from the love and reverence of God." [Century Dictionary, 1895]