"enemy, foe," originally prp. of feogan
"to hate," from P.Gmc. *fijæjan
(cf. O.N. fjandi,
like the O.E. word all prp. forms), from PIE base *pei-/*pi-
"to blame, revile" (cf. Goth. faian
"to blame;" see passion
). As spelling suggests, it was originally the opposite of friend,
but the word began to be used in O.E. for "Satan" (as the "enemy of mankind"), which shifted its sense to "diabolical person" (c.1220). The old sense of the word devolved to foe,
then to the borrowed enemy.
For spelling with -ie-
Meaning "devotee (of whatever is indicated)," cf. dope fiend,
is from 1865.