'Humankind' is not really an adjective. It's a noun, like 'mankind', meaning much the same: 'human-kind', first recorded in the 18th century, is the whole human race. It has the advantage of being gender-neutral, whereas 'mankind' suggests that only men count. Oxfam, though, is breaking new ground by using 'humankind' as an adjective. We can surmise that to be 'humankind' is not only to be kind to other humans, it is to share in their common humanity. The line contrasts with the agency's previous slogan, 'Working for a fairer world'. There, Oxfam was doing all the work, and campaigning for justice. This is less specific, more geared to the range of Oxfam's supporters, from those toiling in disaster zones to those dropping bags of unwanted goods at its shops. The line was devised by Oxfam's new agency, RKCR/Y&R, and first used in April as part of a 'brand refresh', along with an animated TV ad. It's supposed to say that we're all human beings, all in it together: and it does. The grammatical unorthodoxy is what makes it memorable - which is what slogans need to be.