Sustain: to give support or relief to; to supply with sustenance; keep up, prolong. What happens when you pour out and never refill? What happens when you start a fire but never add any more fuel to it? You run dry, you burn out. Learn how to supply with sustenance.Sustain: to give support or relief to; to supply with sustenance; keep up, prolong. What happens when you pour out and never refill? What happens when you start a fire but never add any more fuel to it? You run dry, you burn out. Learn how to supply with sustenance.