'Sustenance' is Latin for 'what holds you up' — the support that keeps you alive. From 'tenere' (to hold).
Food and drink regarded as a source of strength; nourishment; the maintaining of someone or something in life or existence.
From Old French 'sustenance' (nourishment, support), from Latin 'sustinentia' (endurance, self-control, patient bearing), from 'sustinēre' (to hold up, to support, to endure, to withstand), from 'sus-' (variant of 'sub-,' up from below) and 'tenēre' (to hold, to keep), from PIE *ten- (to stretch, to hold firm). The PIE root *ten- is pervasive: it underlies 'tendon, tense, tender, tenor, tenacious, tenant, obtain, retain, contain,' and Greek 'teinō' (I stretch). The image in 'sustain' is physical and precise: something presses down
The modern buzzword 'sustainable' — as in sustainable development, sustainable farming — is built from the same root. Something sustainable can be 'held up from below' indefinitely. The environmental movement adopted a word whose Latin etymology already contained the concept of ongoing support: if you sustain something, you hold it up; if it is sustainable, it can be held up without collapse.