'Sustain' is Latin for 'hold up from below' — from 'sub-' + 'tenere' (to hold). Support from beneath.
To strengthen or support physically or mentally; to keep something going over time; to undergo or suffer (a loss or injury).
From Old French 'sustenir' (to hold up, support, endure), from Latin 'sustinēre' (to hold up, to bear from below, to endure, to maintain), from 'sub-' (from below, up from under) + 'tenēre' (to hold, to keep, to grasp), from PIE *ten- (to stretch, to pull, to extend). The image is physical: to sustain is to hold something up from below, to keep it from falling. PIE *ten- is one of the most productive roots
The '-tain' in sustain, obtain, maintain, pertain, attain, contain, retain, detain, and entertain all comes from Latin 'tenēre' (to hold). Each prefix changes the kind of holding: 'sus-' (from below), 'ob-' (toward), 'main-' (by hand), 'per-' (through), 'at-' (to), 'con-' (together), 're-' (back), 'de-' (down), 'enter-' (among).