Seeing the great investment opportunity, he did not think twice and bought the stocks right away.

not 是否定词，表示"不"或"没有"。 think 的意思是"思考、考虑"。 twice 的意思是"两次"，字面意思是"两倍"。 not think twice 是一个固定短语，字面意思是"不考虑两次"，引申为"毫不犹豫"的意思。