Wiktionary
last but one
a. (context now chiefly UK English) penultimate, next to last, immediately preceding the end of a sequence, list, etc. alt. (context now chiefly UK English) penultimate, next to last, immediately preceding the end of a sequence, list, etc.
Usage examples of "last but one".
Heavy broad-backed old-fashioned mahogany and horsehair chairs, not easily lifted, obsolete tables with spindle-legs and dusty baize covers, presentation prints of the holders of great titles in the last generation, or the last but one, environ him.