Origins
Retirement is a retreat. The word retire comes from Middle French retirer — re- ('back') plus tirer ('to draw, to pull'). To retire was to pull back, and its first English use was on the battlefield.
In 16th-century military language, to retire meant to withdraw forces from an engagement. It was not surrender — it was tactical repositioning, a calculated pulling back. Generals retired their troops to regroup, to find better ground, to fight another day.
The sense of leaving one's career arrived in the 17th century, and the metaphor is telling. To retire from work is to execute a strategic withdrawal from the field of labour. Society framed the end of a working life not as defeat but as an honourable retreat.
Later History
The older spatial meaning persists in the phrase 'to retire for the night'. In great houses, one retired to one's chamber — withdrew from company to a private room. Jane Austen's characters retire after dinner with the ease of a military manoeuvre.
The French tirer ('to draw, to pull') appears in other English borrowings. A tirade was originally a long pull — a drawn-out speech. The tirer in retire and the draw in withdraw mirror each other perfectly: both describe the same backward motion in different languages.