Lewis & Short

paenultĭmus (pēn-), a, um, adj. [paene-ultimus],

  1. I. the last but one (post-class. for proximus a postremo, Cic. Or. 64, 217): paenultima meta November, Aus. Ecl. Quotae Cal. Sint Mens. 12.
  2. II. Subst.: pae-nultĭma, ae, f. (sc. syllaba), the penultimate syllable, the penult: paenultimam circumflectere, Gell. 4, 7, 2.