The derivation of coom is uncertain. Older usage relates to a timber frame for an arch or bridge, with the implication of a rounded or domed shape.  The earliest example in the Dictionary of the Scots Language (www.dsl.ac.uk) is from the Registrum Domus de Soltre, ed. D Laing (1646): “For timber to be the coomesyle, and scaffalding for masones”. Here it appears as part of the compound coomsyle, meaning the timbering of an arch or arched ceiling. 

Many nineteenth-century examples clearly refer to wooden framing for structures on a fairly large scale. These are typically bridges, as in this quotation from Alexander Balfour's Weeds and Wild Flowers (1830): “See, there's the coom o' the laird's new brig down the burn!” or large arches, as in this extract from a letter by Sir Walter Scott: “There will be no difficulty in erecting a scaffolding [in Melrose Abbey ruins] strong eno to support the weight of an interior arch or coom as we call it of wood” (1822).  

Coom could also refer to a coffin lid, from its arched shape, as illustrated by this quotation from Robert Chambers' The Book of Days (1863): “Some surgeon apprentices rudely . . . broke down part of the cooms, or sloping roof of the coffin”.

The adjective coomed is explained by James Barrowman in his Glossary of Scotch Mining Terms (1886): “The roof of a mine … is said to be coomed when it is arch shaped”.  Later examples of coom refer to more domestic contexts, meaning a sloping part of a ceiling, and coomed is sometimes shortened to coom, as in “An attic room with combe ceiling” from the St Andrews Citizen (1936).  So from referring to the underlying framework for an arch or ceiling, coom can now be used as an adjective describing the ceiling itself.  

Scots Word of the Week is written by Ann Ferguson of Scottish Language Dictionaries