The Scotch system of building, like a barrack, all the outhouses of a farmstead, as the byres, stables, barns, etc. The farm menservants live here. (Gaelic, bothag, a cot or hut, our booth.)
“The bothie system prevails, more or less, in the eastern and north-eastern districts.” —J. Begg, D.D.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894