Sentence:  ... we took our courage in both hands and started at the first break in the downpour. The valley was choked with mist, and the road in a state of unutterable <WSD>slabbiness</WSD>: yet our enterprise was soon rewarded, for the weather had done its worst in the darkness, and the sunshine brought the vapours steaming up out of the meadows and banished them with the clouds across the summits of the hills. 
Meaning of the word marked with <WSD> tags:  The quality of being slabby; sliminess, muddiness.