Sentence:  Now she kept herself protectively ready to laugh again and sure enough he pointed at her with his index 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 <WSD>accelerator</WSD> , just as his hand did when he worked . 
Meaning of the word marked with <WSD> tags:  a valve that regulates the supply of fuel to the engine