I bring an unaccustomed wine
Unaccustomed wine
Not her usual offering—she's bringing something special, unfamiliar. The word 'unaccustomed' suggests this is an extraordinary attempt at comfort or healing.
Crackling with fever
The sound of dry, burning lips. 'Essay' means 'attempt'—they try to drink but can barely manage it.
Superfluous cold
Beyond cold, excessively cold—the cold of death, not fever. The temperature has inverted while she looked away.
Frost beneath the mould
'Mould' is dirt, earth. She's talking about corpses frozen in graves for ages—that's how impossible warming these lips would be.
Unto the little
Matthew 25:40—'Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me.' Christ's words about serving the humble.