What the Victorians knew as the sack coat first appeared in France at the end of the 1840's and quickly spread to England and America, becoming very popular in the East by the mid-1850's. Intended for extremely informal occasions, sack coats soon became working and business wear for skilled workers and clerks. By the end of the 1850's the U.S. Army had adopted a military version of the sack coat as fatigue wear. By the 1870's civilian sacks were being worn as general purpose outdoors and working jackets by many people out west.
Despite what you may have read, they are not called "sack coats" because they are oversized, loose, or otherwise fit like a sack. Sack, sac, sacque, etc. all refer to the way the back of the jacket is cut; i.e. "sack cut". This simply means the back is formed of two pieces only, cut relatively straight down, instead of being made up of four curved pieces with hidden pockets in the tails as on more formal and traditional coats such as tail coats, morning coats, and frocks. Some tailoring manuals of the 1850's and 1860's refer to the sack coat by other names, but it's the same garment. Length of skirt and sleeve, number and style of pockets, collar, lapels, and the cut of the front skirt were the elements of changing style in the sack coat from 1850 to 1900. At all times in the period, sack coats were made in "close cut", "full cut", "single breasted", and "double breasted" versions
In 1900, Brooks Brothers introduced its "No. 1 Sack Suit." This version of the sack became the inconic model of the American upper class RTW version of the English lounge suit, and the basis for the collegiate style variants of other multi-store makers like Chipp and Press, as well as all the individual boutique shops, often adjacent to elite colleges and prep schools, that serviced the social groups wearing this look. In the late 1950's and early 1960's, this look became entwined with American confidence and optimism, and a shared sense of destiny among social classes that had been more stucturally divided in the past: many wore it.
Today, it is the preserve of the dwindled social class that wore it originally and of mid-20th century clothing re-enactors.