The Spanish word 'losa', meaning 'flagstone' or 'slab', traces its origins back to Celtic roots. It comes from the Celtic word 'lausia', meaning 'flat stone', which was borrowed into Late Latin preserving the same form and meaning. The word then evolved into modern Spanish 'losa', maintaining its connection to flat, stone surfaces.

A common related Spanish word is 'loseta', which is a diminutive form of 'losa' meaning 'tile' or 'small slab'. You might also encounter 'enlosar', which means 'to pave' or 'to tile', directly derived from 'losa'.

While English doesn't have many direct cognates from this Celtic root, you might find it interesting that some technical terms in geology and architecture use 'loess' (a type of sediment) and 'losenge' (a diamond-shaped flat object), which may share distant connections to the same concept of flat stones or surfaces, though these connections are tentative.