The story
From figure + 9. From the shape of the glyph "9", and creating a figure akin to that shape.
Etymology from Wiktionary, CC BY-SA 4.0.
figure 9
Today it means: A technical climbing technique used in ice climbing to gain a respite and lessen slack and distance from the undersurface/ceiling which one is climbing underneath, which appears like a number "9" in shape, with one arm anchoring the climber to the surface while hanging upside-down, and one leg wrapped around that arm, forming a figurative "9" in shape. Full definition →