Sentence:  Without reason, against prudence, and at a moment's notice, he fell in love with a frivolous, golden-haired girl who used to tear about Simla Mall on a high, rough <WSD>waler</WSD>, with a blue velvet jockey-cap crammed over her eyes. 
Meaning of the word marked with <WSD> tags:  A breed of light saddle horse from Australia, once favoured as a warhorse.