Simple one

______________ is an example of slang serving as a mild profanity. The expression can be used in an angry, wry, sarcastic, cynical, exasperated, or even joking tone. The expression dates to at least the late 19th century, although according to Mark Twain it was already old in 1850, and likely originates with the ancient Christian three-letter symbolism, the Christogram.
Using __________ as an oath has been common for many centuries, but the precise origins of the letter ___ in this expression are obscure. While many explanations have been proposed, some serious and many humorous, the most widely accepted derivation is from the divine monogram of Christian symbolism. The symbol, derived from the first three letters of the Greek name of Jesus is transliterated giving rise to the backronym 'Iesus Hominum Salvator'.