Old English gefea, gefa "foe, enemy, adversary in a blood feud" (the prefix denotes "mutuality"), from adjective fah "at feud, hostile," also "guilty, criminal," from Proto-Germanic *faihaz (source also of Old High German fehan "to hate," Gothic faih "deception"), perhaps from the same PIE source that yielded Sanskrit pisunah "malicious," picacah "demon;" Lithuanian piktas "wicked, angry," peikti "to blame." Weaker sense of "adversary" is first recorded c. 1600.
Old English synn "moral wrongdoing, injury, mischief, enmity, feud, guilt, crime, offense against God, misdeed," from Proto-Germanic *sundiō "sin" (source also of Old Saxon sundia, Old Frisian sende, Middle Dutch sonde, Dutch zonde, German Sünde "sin, transgression, trespass, offense," extended forms), probably ultimately "it is true," i.e. "the sin is real" (compare Gothic sonjis, Old Norse sannr "true"), from PIE *snt-ya-, a collective form from *es-ont- "becoming," present participle of root *es- "to be."
The semantic development is via notion of "to be truly the one (who is guilty)," as in Old Norse phrase verð sannr at "be found guilty of," and the use of the phrase "it is being" in Hittite confessional formula. The same process probably yielded the Latin word sons (genitive sontis) "guilty, criminal" from present participle of sum, esse "to be, that which is." Some etymologists believe the Germanic word was an early borrowing directly from the Latin genitive. Also see sooth.
Sin-eater is attested from 1680s. To live in sin "cohabit without marriage" is from 1838; used earlier in a more general sense. Ice hockey slang sin bin "penalty box" is attested from 1950.
Most Indo-European words for "personal enemy" cover also "enemy in war," but certain languages have special terms for the latter, such as Greek polemioi (distinct from ekhthroi), Latin hostis, originally "stranger" (distinct from inimicus), Russian neprijatel' (distinct from vrag). Russian vrag (Old Church Slavonic vragu) is cognate with Lithuanian vargas "misery" (see urge (v.)), and probably is related to Proto-Germanic *wargoz, source of Old Norse vargr "outlaw," hence "wolf;" Icelandic vargur "fox;" Old English wearg "criminal, felon;" which likely were the inspirations for J.R.R. Tolkien's warg as the name of a kind of large ferocious wolf in "The Hobbit" (1937) and "Lord of the Rings." Related: Enemies.