What does fūsterna mean in Latin?

fūsterna, a noun, means “the upper part of a fir tree”.

What is the genitive of fūsterna?

The genitive singular is fūsternae. fūsterna is feminine, first declension.

noun · femininefirst declension

fūsterna f (genitive fūsternae)

fusterna, ae, f. contr. from fusterina, sc. pars, from fustis,

I. the upper part, knotty part of a fir - tree, Plin. 16, 39, 76, § 196; Vitr. 2, 9.

Probably from fustis.