1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): spare tire
1. a thickness in the waist; a roll of fat around one’s waist. • I’ve got to get rid of this spare tire. • The spare tire started when I was twenty-six. 2. an unneeded person; an unproductive person. • Gary is a spare tire. Send him home. • You spare tires over there! Get to work.
McGrawhill's American Idioms And Phrasal Verbs