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Allons-y, according to the Tenth Doctor, was French for "Let's go". (TV: Midnight) It was one of the Tenth Doctor's favourite sayings; he especially wanted to say, "Allons-y, Alonso", (TV: Army of Ghosts) which he eventually said to Alonso Frame. (TV: Voyage of the Damned) The Twelfth Doctor once claimed that he was able to say it due to his precise control over the TARDIS translation circuits. (COMIC: Four Doctors)
The Tenth Doctor yelled the phrase as he, along with twelve other incarnations, combined the power of their TARDISes to use a stasis cube to freeze Gallifrey in a pocket universe. This, combined with the Eleventh Doctor's "geronimo!", came to the irritation of the War Doctor. (TV: The Day of the Doctor)
The Doctor described it to Addams as a word of consolation to the soul in times of need. (TV: The End of Time) He was last heard using it during his final reward, as he proceeded to flee from the Gentlemen of the Dice with George Litefoot. (AUDIO: The Jago & Litefoot Revival)