a fine drizzle, a 'fine roosty rain'. Norse rus and rusk, fine drizzle of rain or snow. The 't' in roost has apparently been added on analogy of Scots, roost. In the North Ronaldsay form roostanhoger, roostan may be a pres. participle of a verb 'to roost', or it may simply be the noun plus 'and'. With hoger, see also bagger.