Sentence:  Now she kept herself protectively ready to laugh again and sure enough he pointed at her with his <WSD>index</WSD> finger and said `` Toot '' ! once more and roared off into the fog , his foot evidently surprising him with the suddenness with which it pressed the accelerator , just as his hand did when he worked . 
Meaning of the word marked with <WSD> tags:  the finger next to the thumb