The story
Coined by mathematicians Meysam Alishahiand and Hossein Hajiabolhassan in 2014 for the 2015 paper "On the chromatic number of general Kneser hypergraphs" as a blend of alternating + chromatic on the grounds that the chromatic bound is given by a longest subsequence in which every pair of consecutive terms differs (loosely, the subsequence "alternates").
Etymology from Wiktionary, CC BY-SA 4.0.