Sacre Bleu!

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Sacrebleu is an old French swearword, meant as a cry of surprise or anger. It would be equivalent to gosh or by Jove.

In French, sacrebleu or sacredieu is always written as one word without accent, the 'e' in the middle being pronounced like a faint and short 'eu' (ə). In English though, the phrase is often written with two words sacre bleu.

Most likely, it comes from old blasphemous curses relating to God, used from the late Middle-Age (some are attested as early as the 12th century) to the 14th at the latest, with many variants: morbleu or mordieu, corbleu, palsambleu, jarnidieu, tudieu, respectively standing for mort [de] Dieu (God's death), corps [de] Dieu (God's body), par le sang [de] Dieu (by God's blood, the two latters possibly referring to the Eucharistic bread and wine), je renie Dieu (I deny God), tue Dieu (kill God)... Those curses may be compared to the old english [God']sdeath, sblood, struth or zounds (God's wounds).

They were considered so offensive that Dieu was sublimated into the neutral syllable bleu which sounds similarly, but doesn't mean anything in that case.

The verb sacrer has several meanings, including to crown, to anoint, and nowadays to name someone [champion, best actor, etc]. But here, it's probably the old meaning, rarely used in France but more common in French Canada, of swear, curse. Therefore, sacrebleu would be in modern French je sacre par Dieu and in English I curse by God.

Some think that the value of the word blue is used to designate the blue of the vein. As opposed to an artery, carrying red blood, a vein is flowing back to the hart. Deoxygenated blood is to represent the empty or evil. "Sacre bleu" is any omen of bad news or schocking ill fortune.

Nowadays those words are totally obsolete in French, unless you want to sound "mediaeval" or "classical". Nevertheless, they are still in the modern dictionaries.

It is often considered in the English-speaking world as a quintessential French phrase. Written with two words (sacre bleu!), it has been popularized by Agatha Christie's belgian hero Hercule Poirot.