Source: Wiktionary — "sisu"

Title: sisu (Wiktionary entry, covering both the Finnish-language headword and a separate English-language loanword section)
Publication: Wiktionary (en.wiktionary.org)
URL: https://en.wiktionary.org/wiki/sisu
Type: tertiary (crowd-sourced dictionary); stance neutral
Accessed: 2026-09-03

Extract

Etymology: from sisä- ("inner") + -u. Originally referred to one's physical interior (compare sisus, sisusta), hence the meaning "core; spirit" and thus "courage, determination." Cognate with Estonian sisu ("content") and — as a semantic (not etymological) parallel — English "guts."
Part of speech: noun.
Senses given: (1) "perseverance, doggedness, stubbornness, grit, stamina, spunk, determination, courage, guts, tenacity" — an unwavering strength to keep going when things are difficult; (2) sisu in cultural context, often with the attributive suomalainen ("Finnish"): strength of will in adversity, regarded as integral to Finnish culture; (3) "temper" — a tendency toward a certain mood type, especially a bad or defiant one.

Definition: "Strength of will in the face of adversity; grit, perseverance or determination, regarded as an integral part of Finnish culture."
Etymology note: borrowed from Finnish sisu, documented in English since at least 1940.
Citations quoted on the page:
1. Time magazine, 8 January 1940: "Last week the Finns gave the world a good example of sisu by carrying the war..." (the Winter War, which began 30 November 1939).
2. Norma J. Livo & George O. Livo, 1999: "People living in northern climates need plenty of sisu."
3. Angela Duckworth, 2016: "We agreed there was a pressing need for a systematic investigation of sisu, how Finns..." — Duckworth is the psychologist and author of Grit: The Power of Passion and Perseverance (2016); this citation places sisu directly inside the modern English-language "grit" research conversation she is best known for.
4. Katie Falkowski, 2016: "Life requires all of us to have a certain level of sisu when things get tough..."

The page does not give a full citation (publication/book title) for the Livo & Livo 1999 or Falkowski 2016 quotations — the authors and approximate dates are recorded, not the specific works.
The Finnish section's senses are Wiktionary's own tertiary gloss of the Finnish word, not a primary Finnish lexicographic source (see the word page's Gaps for the still-unconfirmed Kielitoimiston sanakirja / Kotus lead).