Why? Because. The most terrible of motives, the most unanswerable of retorts — Because.

[Pourquoi ? Parce que. Le plus terrible des motifs et la plus indiscutable des réponses: Parce que.]

Victor Hugo (1802-1885) French writer

Les Misérables, Part 4 “St. Denis,” Book 6 “Little Gavroche,” ch. 1 (4.6.1) (1862) [tr. Hapgood (1887)]
    (Source)

On Mme Thenardier hating her sons.

(Source (French)). Alternate translations:

Why? Because. The most terrible of motives and the most unanswerable of responses: Because.
[tr. Wilbour (1862)]

Why? because she did. The most terrible of motives and most indisputable of answers is, Because.
[tr. Wraxall (1862)]

Why? Because. The most terrible and unanswerable of reasons.
[tr. Denny (1976)]

Why? Because. The most terrible of motives and the most unanswerable of responses: Because.
[tr. Wilbour/Fahnestock/MacAfee (1987)]

Why? Because. The most terrible of motives, the most indisputable of responses. Because.
[tr. Donougher (2013)]