Flab about the waist; a certain embonpoint; bulge(1925+)
A superfluous and unwelcome person: I didn't come, because I knew I'd be a spare tire in that crowd(1940s+)
A tedious person; a bore (1940s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with 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.
[ ; mid-1900s