备胎

备胎	bèi tāi	spare tire / (slang) fallback guy (or girl)
备	bèi	variant of 备
备	bèi	(bound form) to prepare; to equip / (literary) fully; completely
胎	tāi	fetus / classifier for litters (of puppies etc) / padding (in clothing or bedding) / womb carrying a fetus / (fig.) origin / source / (loanword) tire