"last but one," 1530s, abbreviation of penultima. As a noun from 1570s as "last day but one of a month;" grammatical sense of "last syllable but one of a word" is by 1828.
Entries linking to penult
penultima n.
"last syllable but one of a word or verse, a penult," 1580s, from Latin pænultima (syllaba), "the next to the last syllable of a word or verse," from fem. of Latin adjective pænultimus "next-to-last," from pæne "almost" (a word of uncertain origin) + ultimus "final" (see ultimate).