Subsist, v. i. [imp. & p. p. Subsisted; p. pr. & vb. n. Subsisting.] — Webster enters it under “subsist”
Etymology: [L. subsistere to stand still, stay, remain alive; sub under + sistere to stand, to cause to stand, from stare to stand: cf. F. subsister. See Stand.]
1. To be; to have existence; to inhere. And makes what happiness we justly call, Subsist not in the good of one, but all. Pope.
2. To continue; to retain a certain state. Firm we subsist, yet possible to swerve. Milton.
3. To be maintained with food and clothing; to be supported; to live. Milton. To subsist on other men's charity. Atterbury.
1 other Webster entry under this spelling
Subsist, v. t.
To support with provisions; to feed; to maintain; as, to subsist one's family. He laid waste the adjacent country in order to render it more difficult for the enemy to subsist their army. Robertson.
Shades of meaning
Fernald has no entry of its own for subsisting; he lists it under alive.
He lists it without comment; the entries above explain its neighbors.