  The dust production in debris discs by grinding collisions of planetesimals
requires their orbits to be stirred. However, stirring levels remain largely
unconstrained, and consequently the stirring mechanisms as well. This work
shows how the sharpness of the outer edge of discs can be used to constrain the
stirring levels. Namely, the sharper the edge is the lower the eccentricity
dispersion must be. For a Rayleigh distribution of eccentricities ($e$), I find
that the disc surface density near the outer edge can be parametrised as
$\tanh[(r_{\max}-r)/l_{\rm out}]$, where $r_{\max}$ approximates the maximum
semi-major axis and $l_{\rm out}$ defines the edge smoothness. If the
semi-major axis distribution has sharp edges $e_\mathrm{rms}$ is roughly $1.2
l_{\rm out}/r_{\max}$, or $e_\mathrm{rms}=0.77 l_{\rm out}/r_{\max}$ if
semi-major axes have diffused due to self-stirring. This model is fitted to
ALMA data of five wide discs: HD 107146, HD 92945, HD 206893, AU Mic and HR
8799. The results show that HD 107146, HD 92945 and AU Mic have the sharpest
outer edges, corresponding to $e_\mathrm{rms}$ values of $0.121\pm0.05$,
$0.15^{+0.07}_{-0.05}$ and $0.10\pm0.02$ if their discs are self-stirred,
suggesting the presence of Pluto-sized objects embedded in the disc. Although
these stirring values are larger than typically assumed, the radial stirring of
HD 92945 is in good agreement with its vertical stirring constrained by the
disc height. HD 206893 and HR~8799, on the other hand, have smooth outer edges
that are indicative of scattered discs since both systems have massive inner
companions.
