Context: utility theory
A util is commonly used as a "unit" for an individual's utility.
However, utils are only a relative measure of utility, not an absolute one, unlike other units of measure. This is because utility only measures preference between two things, and not as a absolute measure of happiness. For example, while the phrase "1 metre" is meaningful as a unit of length, the phrase "1 util" is absolutely meaningless unless it is compared to something else.
The absolute scale of utility is determined by the form of the utility function. Thus it is quite possible for utility to be negative while still being very happy over something else. As an example, the negative exponential utility function always produce a negative value, whereas the logarithmic utility function can produce both positive and negative values.