Loath - loth

These two spellings are unnecessary. They are both representations of the same adjective, which has the meaning ‘unwilling’, ‘disinclined’ or ‘averse to’. The Oxford Dictionary of American Usage and Style starts its entry on loath: “Loath (with its needless variant loth) is an adjective meaning “reluctant.”

Loath is typically used in such constructions as “He was loath to leave and get back to his work, but the break was over.” OED cites an eminent lawyer saying “One is loth to believe the similarity is innocent and unintentional”, as an expression of distrust.

A common - if rather old fashioned - elegant way of using loath in the negative, to mean ‘to be willing’ is “He was nothing loath.”

See also loath - loathe.