Old English lað "hated; hateful; hostile; repulsive," from Proto-Germanic *laithaz (cognates: Old Saxon, Old Frisian leth "loathsome," Old Norse leiðr "hateful, hostile, loathed;" Middle Dutch lelijc, Dutch leelijk "ugly;" Old High German leid "sorrowful, hateful, offensive, grievous," German Leid "sorrow;" French laid "ugly," from Frankish (Germanic) *laid), from PIE root *leit- "to detest."
Weakened meaning "averse, disinclined" is attested from late 14c. Loath to depart, a line from some long-forgotten song, is recorded since 1580s as a generic term expressive of any tune played at farewells, the sailing of a ship, etc. Related: Loathness.