Definitions: (1) having a firm and resolute character; stability of purpose; (2) the most substantial or sturdiest part; the foundation

Synonyms: courage, determination

Idiom: Keep a stiff upper lip.

• To succeed in life, you need three things: a wishbone, a backbone and a funny bone. — Reba Nell McEntire (1955) American singer
• You are the age of your spine. You are as flexible as your spine. That transfers to other areas of your life. — Diane Colleen Lane (1965-) American actress