Sentence:  "Poison," said Sherlock Holmes curtly, and strode off. "One other thing, Lestrade," he added, turning round at the door: "‘Rache,' is the German for ‘revenge'; so don't lose your time looking for Miss Rachel." / With which <WSD>Parthian</WSD> shot he walked away, leaving the two rivals open-mouthed behind him. 
Meaning of the word marked with <WSD> tags:  Delivered as if in retreat. ("An allusion to the Parthian battle tactic of firing arrows backwards from horseback while apparently in retreat.")