De Verborum Significatu
Sextus Pompeius Festus
Penus vocatur locus intimus in aede Vestae tegetibus saeptus, qui certis diebus circa Vestalia aperitur.
The innermost place in the temple of Vesta, surrounded with screens, is called the Penus, which is open on certain days around the Vestalia.
Translation by Jane W. Crawford, © 2001.