Sentence:  Many very fleet horses, when overdriven, adopt a disagreeable gait, which seems to be a cross between a pace and a trot, in which the two legs of one side are raised almost but not quite, simultaneously. Such horses are said to <WSD>single</WSD>, or to be single-footed. 
Meaning of the word marked with <WSD> tags:  To take the irregular gait called singlefoot.