It seems a bit rude to report this as a "bug" on bugtracker, and I suspect there is a different mechanism for discussing words for inclusion/exclusion in the default dictionary. But my google fu is failing me in this instance. At any rate, "unbeknownst" is not considered valid by the dictionary in my 58.0.2 Firefox build (on Ubuntu if that makes any difference). Seems to be a valid adjective of "unbeknown" according to Merriam Webster. Please let me know where this should be reported/discussed.