  This work concerns a generalization of property (RD) from discrete groups to
twisted \'etale groupoids equipped with a length function. We show that, under
the assumption that the \'etale groupoid is principal, twisted property (RD) is
equivalent to polynomial growth. This generalizes a result of Chen and Wei
concerning rapid decay for metric spaces with bounded geometry. Additionally,
some permanence properties of groupoid (RD) are established.
