Root & Form
خ س ر
الخُسرُ والخُسرانُ: النَّقصُ والضَّياعُ — ذَهابُ رأسِ المالِ
Khusr / Khusrān: loss, diminishment — the going-away of one's capital and principal.
From Mufradāt al-Rāghib al-Aṣfahānī
الخُسرانُ: انتقاصُ رأسِ المالِ، وقد يُستعمَلُ ذلك في المقتنياتِ الخارجةِ كالمالِ والجاهِ، وفي المقتنياتِ النَّفسانيَّةِ كالصِّحَّةِ والسَّعادةِ والعقلِ والإيمانِ.
Khusr is the diminishment of one's capital. It applies to external possessions such as wealth and status, and to inner ones such as health, happiness, reason, and faith — the loss of anything one has been given.
Grammatical emphasis: لَفِى
The construction inna … la-fī (إِنَّ … لَفِى) is a double emphasis in Arabic — "truly, indeed, most certainly in." The nesting of the human in loss (fī = inside) suggests total immersion: we do not merely face loss; we are surrounded by it on all sides.
The word khusr is used in commerce — a merchant who loses his capital. Our capital is time, health, and ability. The surah says: if you do not actively invest these in the four qualities of Ayah 3, you are already in deficit. The default is loss; the profit requires effort.