Beurre Manie: Flour-Butter Paste

Beurre Manie sauce:  flour-butter paste: a quick sauce thickener.
It means literally butter and flour worked to a paste with fingers.

After you cooked your sauce, and it has been reduced and seasoned and it still needs thickening, you can make it thicker with Beurre Manie. Used for stews such as chicken in red wine, coq au vin, beef bourguignon.

For each cup of sauce, mash 1 tablespoon of butter to a smooth paste with 1 tablespoon of flour. Remove the sauce from heat, whisk in the butter-flour paste, when absorbed, bring to the boil and the sauce will thicken.

Julia Child. The way to cook. Page 143.