Sentence:  I had been calling Nobs in the meantime and was about to <WSD>set out</WSD> in search of him, fearing, to tell the truth, to do so lest I find him mangled and dead among the trees of the acacia grove, when he suddenly emerged from among the boles, his ears flattened, his tail between his legs and his body screwed into a suppliant S. He was unharmed except for minor bruises; but he was the most chastened dog I have ever seen. 
Meaning of the word marked with <WSD> tags:  To go out, leave.