B. Zuanella explains that the surname derives from the place name Trusgne —a locality of the municipality of Drenchia, in the Friuli-Venezia Giulia region— through the Slovenian suffix -njak, and that it originally designated the inhabitant of that village or the person coming from there. The old forms present the suffix -nik (variants Trusnig / Trušnik), which later evolved towards -njak (Trusgnach / Trušnjak), a process typical of the Slovenian dialect of the Natisone valley, comparable to the transformation of surnames such as Dornig into Dorgnach or Bernig into Bergnach. The surname is concentrated today mainly in the municipalities of Drenchia and Grimacco.

E. Rossoni agrees on the toponymic origin and specifies that in the Friulian dialect the corresponding form is trusniaco, noting the Udine area as a zone of presence, with localities such as Cividale del Friuli, Grimacco, Drenchia and San Pietro al Natisone.