spare tire

noun
1.
a tire used or kept available as an emergency replacement on a vehicle.
2.
Informal. excess fat around the waistline.

Origin:
1920–25

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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American Heritage
Idioms & Phrases

spare tire

Fat around one's middle, as in He's determined to lose ten pounds and that spare tire he's acquired. This expression transfers the term for an extra tire carried in cars in case of a flat tire to excess fat around the waist. [Colloquial; mid-1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
Cite This Source
Example sentences
Liposuction is the only way to actually reduce the number of fat cells in your
  spare tire.
The spare tire bolts into a recess offset to the right in the swing-gate, and
  doesn't dip below the bumper line.
It still had the original hose clamps and spare tire which have since been
  replaced along with the original shocks.
There's something vulnerable about an eighteen-wheeler-sized spare tire that
  everyone thinks they can handle.
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