This is a shortened compound of the words "Less" and "bOTHered".
A general negative response or feeling to a situation or question. Eg. A response to "are you having a quiet night in with the mrs?", would be "loth to that". A counterpart could be lance (less chance) or lin (less interested). Is perhaps the most apt of all members of "The Code" developed in Bristol as it actually appears in the Oxford English Dictionary and has a French counterpart which is "boff".