If you do something that's selfish or unsympathetic, it's uncharitable. It would be uncharitable to throw away your unopened bag of chips when your friend just mentioned how hungry she is.
Uncharitable behavior includes things like gossiping meanly about others, refusing to offer someone a ride when you're heading in the same direction, or hoarding your vast amounts of wealth rather than donating some of it to charity. In fact, the noun charity is at the heart of this adjective — as well as "an organization that provides help," it means "voluntarily giving help" and "kindness." When you don't show charity, you're uncharitable.