Back charges, charges brought forward after an account has been made up. -- Back filling (Arch.), the mass of materials used in filling up the space between two walls, or between the inner and outer faces of a wall, or upon the haunches of an arch or vault. -- Back pressure. (Steam Engine) See under Pressure. -- Back rest, a guide attached to the slide rest of a lathe, and placed in contact with the work, to steady it in turning. -- Back slang, a kind of slang in which every word is written or pronounced backwards; as, nam for man. -- Back stairs, stairs in the back part of a house; private stairs. Also used adjectively. See Back stairs, Backstairs, and Backstair, in the Vocabulary. -- Back step (Mil.), the retrograde movement of a man or body of men, without changing front. -- Back stream, a current running against the main current of a stream; an eddy. -- To take the back track, to retrace one's steps; to retreat. [Colloq.]