The story
From an explanation of a principle of fluid dynamics given by E. M. Purcell, referring to the opening and closing motion of a scallop's hinge.
Etymology from Wiktionary, CC BY-SA 4.0.
scallop theorem
Today it means: A theorem which states that to achieve propulsion at low Reynolds number in simple (i.e. Newtonian) fluids, a swimmer must deform in a way that is not invariant under time-reversal. Full definition →