When someone grabs your arm to keep you from crossing the road as a car is speeding down the street, you are forever grateful to them, you owe them your life. In French, vous lui devez une fière chandelle - you owe him/her a proud candle. Depending on the context the expression will translate as owing a big favor, a great deal of gratitude, being indebted to someone, or in everyday language: owing them "big time. "
The French expression is definitely colorful and intriguing. It is also very odd. If it didn't involve a candle, one could say it is actually pretty obscure. And how on earth can a candle be proud?
The expression comes from the Christian tradition. In the Middle Ages, it was to God, the Virgin Mary or other saints that people felt indebted to when they escaped great danger. As a sign of gratitude, it was customary to go to church and burn a candle. At the time, the full expression was devoir une fière chandelle à Dieu - owing a proud candle to God. The expression gradually extended to whoever you felt had saved your life or the life of a close one. You would burn a candle for God to protect them if they ever found themselves in danger too.
As for the adjective proud, it does not refer to any pride in the candle but rather to its size and refinement. Indeed, the more impressive the candle, the deeper the gratitude. The use of the adjective takes its full meaning if one considers the historical context: In those days, candles were not just the only source of lighting, they were also extremely expensive - paraffin and stearin that made manufacturing candles more affordable were discovered in the 19th century. Before that, ordinary citizens had to get by with tallow candles which produced a pungent odor and thick smoke. Expensive and rare beeswax candles were mostly reserved for the use of lords and clergy. Burning a candle in church therefore had a certain price.
Note that the English candle is more often translated as bougie in today's French. Strictly speaking une chandelle is a tallow candle, and une bougie is a wax candle. While Canadian French kept chandelle as its main word for candle, European French mostly use bougie nowadays, and only use chandelle in set expressions. There is also a third word, un cierge, specialized for candles to be lit in a church.
Je dois une fière chandelle à mes instituteurs d'école primaire ; ils m'ont transmis le goût d'apprendre.
I owe a debt of gratitude to my elementary school teachers; they gave me the desire to learn.
Elle a tout fait pour t'aider. Tu lui dois une fière chandelle!
She's done everything to help. You should be forever grateful to her!