What is the X in X-mas?
late 14c., "causing fever;" 1630s, "excited;" 1640s, "having symptoms of fever," from fever + -ish. Earlier in same sense was feverous (late 14c.). Related: Feverishly; feverishness.
feverish fe·ver·ish (fē'vər-ĭsh)adj.
Having a fever.
Relating to or resembling a fever.
Causing or tending to cause a fever.