The story
From humane + wash, following the established pattern of greenwash, pinkwash, and similar forms (which came ultimately from the figurative-extension sense of whitewash as "to slap a paintjob over the truth to hide it and to give a specious nice appearance"); this form also rhymes with brainwash, as does sustainwash.
Etymology from Wiktionary, CC BY-SA 4.0.