When he asked me out, I didn't think twice!

didn't 是 do not 的缩写形式，表示否定。 think 的意思是"思考、考虑"。 twice 的意思是"两次"，在这里 think twice 是固定搭配，字面意思是"思考两次"，引申为"犹豫、再三考虑"。 didn't think twice 这个短语表示"毫不犹豫、不假思索地做某事"。