To emaciate is to make someone extremely thin or very weak. A serious illness can often emaciate a person, leaving them gaunt and frail.
The verb emaciate is much less common than its related adjective, emaciated. Both stem from the Latin emaciare, "make lean, cause to waste away." Whenever a person has become malnourished in a way that's evident just from looking at them, you can use this word: "The ravages of the Irish potato famine emaciated the starving people all across the country, eventually causing a million deaths."