Sentence:  When one is snugly ensconced under several thicknesses of eiderdown, with the frozen water-bottle sending a cracked and mocking leer from the window sill, getting up is the one thing really irrevocable. It becomes the most final of <WSD>Rubicons</WSD>, the most suicidal of bridge-burnings, a leap into an abyss of vaguely dreadful activities,—a fantastic world where people stand on their feet and tie neckties. 
Meaning of the word marked with <WSD> tags:  A limit that when surpassed cannot be returned from, or an action that when taken cannot be reversed.