Other forms: loathingly; loathest
If you are loath to do something, you really don't want to do it. If you are reluctant to go swimming, people will say you are loath to swim — but if they are really mean, they might throw you in the pool anyway.
The adjective loath is used to describe being extremely opposed to something. The term is generally followed by to — "The teacher was loath to let the students turn in papers late, but he made an exception for the girl who had missed class due to illness."