The story
From turtle (“from its half-turtle-shell-like shape”) + curb.
Etymology from Wiktionary, CC BY-SA 4.0.
turtle curb
Today it means: A type of curb and speed bump found on motor racing tracks, having an oblong shape and elevated profile of a turtleshell, used for path and areal denial to delimit track limits, and launch vehicles into the air, blow-out suspensions, rip-out undercarriages, of those who choose to run over it. Full definition →