What is the X in X-mas?
Old English feond "enemy, foe," originally present participle of feogan "to hate," from Proto-Germanic *fijæjan (cf. Old Frisian fiand "enemy," Old Saxon fiond, Middle Dutch viant, Dutch vijand "enemy," Old Norse fjandi, Old High German fiant, Gothic fijands), from PIE root *pe(i)- "to blame, revile" (cf. Gothic faian "to blame;" see passion).
As spelling suggests, it was originally the opposite of friend, but the word began to be used in Old English for "Satan" (as the "enemy of mankind"), which shifted its sense to "diabolical person" (early 13c.). The old sense of the word devolved to foe, then to the imported word enemy. For spelling with -ie- see field. Meaning "devotee (of whatever is indicated)," e.g. dope fiend, is from 1865.
A devotee or user of what is indicated: camera fiend/ dope-fiend/ sex fiend (1865+)verb
To use a choke hold on a robbery victim: They'd take out a bodega, or fiend a few housewives (1980+ Underworld)