Origins
The word 'sustain' entered English in the late thirteenth century from Old French 'sustenir' (later βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ'soutenir'), descended from Latin 'sustinΔre,' meaning 'to hold up,' 'to bear,' 'to endure,' or 'to maintain.' The Latin verb is composed of 'sus-' (a variant of 'sub-,' meaning 'from below' or 'up from under') and 'tenΔre' (to hold), from PIE *ten- (to stretch). To sustain something is literally to hold it up from below β to be the support that keeps it from falling.
Latin 'tenΔre' is one of the most prolific Latin verbs in English vocabulary, appearing in disguised form in an enormous family of words. The key insight is that the '-tain' ending in many common English verbs is a reflex of 'tenΔre,' modified by different prefixes. 'Sustain' (to hold up from below), 'obtain' (to hold toward oneself, to acquire), 'maintain' (to hold by hand, from Latin 'manu tenΔre'), 'pertain' (to hold through, to relate to), 'attain' (to hold to, to reach), 'contain' (to hold together), 'retain' (to hold back), 'detain' (to hold down or away), and 'entertain' (to hold among, to keep engaged) all descend from 'tenΔre' with different Latin prefixes.
Beyond the '-tain' family, 'tenΔre' produced 'tenant' (one who holds land), 'tenure' (the holding of an office or property), 'tenacious' (holding firmly), 'tenet' (a thing held β a principle), 'tenor' (the general course or holding β also the singing voice that 'holds' the melody), 'tenable' (able to be held), and 'lieutenant' (one who holds the place of another, from French 'lieu' + 'tenant'). The legal term 'tenement' (a holding, a piece of property) comes from the same source.
Figurative Development
The word 'sustain' entered English with several senses that it retains. To sustain physically: 'food sustains life,' 'the pillars sustain the roof.' To sustain emotionally or morally: 'hope sustained them through the ordeal.' To sustain in the sense of undergoing: 'he sustained serious injuries.' To sustain legally: 'the court sustained the objection' (held it up as valid). Each sense preserves the core metaphor of holding something up.
The modern derivative 'sustainable' has become one of the defining words of early twenty-first-century discourse. 'Sustainable development,' 'sustainable agriculture,' 'sustainable energy' β the word appears in virtually every discussion of environmental and economic policy. The concept entered mainstream usage following the 1987 Brundtland Report, which defined sustainable development as 'development that meets the needs of the present without compromising the ability of future generations to meet their own needs.' The etymological image is apt: sustainable practices are ones that can be held up β sustained β indefinitely, without the supports giving way.
The musical term 'sustain' (to hold a note for its full duration, or the sustain pedal on a piano that allows strings to continue vibrating) preserves the literal 'holding' sense most directly. A sustained note is one that is held β kept vibrating β rather than allowed to decay. The sustain pedal on a piano lifts all the dampers from the strings, allowing them to resonate freely, creating the instrument's most characteristic wash of sound.