Thought for the Day, which is the Radio 4 Today programme's 'God spot', was devoted today to minced oaths (listen here for another week - scroll through to about 1:47:30). The speaker, Clifford Longley, referred to a topical news story about footballers swearing at and abusing each other, which has landed them in court. Longley talked of how once, the most abusive swear words were considered those with religious connotations (since it meant you were breaking the Commandment 'Thou shalt not take the name of the Lord thy God in vain', and committing blasphemy). In order to keep on the right side of the law, people changed the religious swear words to something milder, and these are minced oaths, the verb 'to mince' here meaning to moderate one's language for the sake of prudence or decorum'.
Examples of minced oaths used by Clifford Longley included 'bloody', which he said was a contraction of 'by our Lady'. The OED thinks that this suggested origin is unlikely. He also mentioned Cripes! and Crikey! (contractions of Christ), Blimey! (= God blind me) and Zounds! (= by God's wounds, a reference to the Crucifixion).