Source: Wikipedia — "Sisu (film)"

Extract

The 2022/2023 Finnish action film Sisu, written and directed by Jalmari Helander, is set in late 1944 during the Lapland War. Its protagonist, Aatami Korpi, a former Finnish Army commando turned gold prospector, is attacked by a Waffen-SS platoon and violently fights back.
Director's stated influences: Helander has named the 1982 film First Blood as a stylistic reference for the action sequences, and Simo Häyhä — the real Finnish sniper who fought in the actual Winter War (1939–1940) and is popularly the most-cited human embodiment of sisu in Finnish military mythology — as the model for the protagonist's near-unstoppable characterization.
Critical reception was strong (94% positive per Rotten Tomatoes' aggregation of 181 reviews; Metacritic 70/100); the film grossed $14.3 million worldwide, the most successful Finnish film at the US box office to that point.
The article itself does not discuss the word/concept "sisu" beyond using it as the film's title — no quoted director's statement in this article explains the title choice or connects the film's stylized ultraviolence to the word's academic or popular meaning.

This project could not directly access a specific published critique connecting the film to "toxic masculinity" (one such piece, at bleedingfool.com, returned HTTP 403 to WebFetch this session) — flagged as an inaccessible lead, not a confirmed claim, consistent with this project's standing practice of not citing WebSearch-synthesized summaries of blocked pages as sourced fact.
Whether Helander or any credited screenwriter has stated in an interview why the film is titled after this specific word, rather than simply "sisu" as generic Finnish military vocabulary, is unconfirmed.