backbone

[1]	anchor , spine , spinal column , mainstay , keystone , linchpin , lynchpin
[2]	grit , mainstay , chief/main support , guts , moxie , buttress , pillar , sand , gumption
[3]	spinal column , resoluteness , sturdiness , vertebral column , spine , firmness , determination , back , rachis , strength (of character) , mettle , purposefulness , resolution , courage , fortitude , resolve , will , will-power , strength , stability , stamina , staying power , grit
[4]	spine