I came across the following sentence in today's NPR news:
In 2011, boomers start turning 65, the age when Americans traditionally stop working and kick back.
A dictionary at hand gives the definition of kick back as "1. (literally) kick back, retort. 2. pay rebate. 3. relapse into illness. 4. get relaxed, put oneself at home."
Interpretations 1 through 3 are understandable to me. But I don't understand why 'kick back' came to mean definition 4, "get relaxed, put oneself at home." What is the etymology for this expression?