The kanji for 'ninja' places a blade (刃) over a heart (心) — calm under threat, literally spelled out.
A person skilled in the Japanese art of ninjutsu; a covert agent or mercenary in feudal Japan.
From Japanese 'ninja' (忍者), a compound of 'nin' (忍, to endure, to bear, to conceal, to persevere) + '-ja' or '-sha' (者, person, practitioner). A ninja is literally an 'enduring person' or 'concealing person' — one trained to persist through hardship and remain unseen. The kanji 忍 is itself a compound: 刃 (ha, blade) positioned over 心 (kokoro, heart/mind) — a blade poised above the heart, evoking the capacity to remain completely
The kanji for 'ninja' (忍者) is deeply poetic. The character 忍 (nin, endure/conceal) is composed of 刃 (yaiba, blade) placed above 心 (kokoro, heart). The visual metaphor: a blade held over the heart — the ability to remain still and endure when a blade threatens. A ninja is, at the character level