"impious or profane speaking of God or sacred things," early 13c., from Old French blasfemie "blasphemy," from Late Latin blasphemia, from Greek blasphēmia "a speaking ill, impious speech, slander," from blasphēmein "to speak evil of."
The second element of that is phēmē "utterance" (from PIE root *bha- (2) "to speak, tell, say"); the first element is uncertain, perhaps it is 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]
To laugh at the pretensions of Mohammed in Constantinople is blasphemy. To say in St Petersburg that Mohammed was a prophet of God is also blasphemy. There was a time when to acknowledge the divinity of Christ in Jerusalem was blasphemy. To deny his divinity is now blasphemy in New York. Blasphemy is to a considerable extent a geographical question. [Robert G. Ingersoll, quoted in "Discussions," 1902]
updated on October 14, 2022