faithfully; loyally ...a1400
from fay (n. religious belief) + -fully
FIRST DOCUMENTED USE
a1400 - see EXAMPLE below
"...They are enbuschede on blonkkes, with baners displayede,
In ȝone bechen wode appon the waye sydes.
Thay hafe the furthe forsette all of þe faire watyre,
That fayfully of force feghte vs byhowys;
For thus vs schappes todaye, schortly to tell,
Whedyre we schone or schewe—schyft as þe lykes..."
From: Morte Arthure, or the Death of Arthur