c. 1600, from Late Latin profanitas, from Latin profanus (see profane (adj.)). Extended sense of "foul language" is from Old Testament commandment against "profaning" the name of the Lord.
Blasphemy, Profanity, agree in expressing the irreverent use of words, but the former is the stronger, and the latter the wider. Profanity is language irreverent toward God or holy things, covering especially all oaths that, literally interpreted, treat lightly the attributes or acts of God. Blasphemy is generally more direct, intentional, and defiant in its impiety, and is directed toward the most sacred things in religion. [Century Dictionary, 1895]