What is the meaning of "whittle"?

Sam resumes walking, still pointed south, heading God knows where. I stay where I am, guilt, fear, and exhaustion holding me in place. I can’t remember the last time I had a full night’s sleep. It was before Sam showed up, I know that much. Her arrival has whittled my rest down to nothing. I don’t see that changing anytime soon. I envision weeks of sleeplessness, my nights disrupted by dreams of Sam, of Rocky Ruiz, of Lisa being held down while her wrists are slit

Choose DictionaryBanglaCambridgeCollinsLongman