If you read Martin Eden carefully, you wuld notice him notice O.Weininger, so most significant work of this person is "Sex and Character". Etimlogy of word "henid" comes from Ancient Greek ἕν (hén, “single thing”) +‎ -id, because it cannot at this stage be separated into thought and feeling. hense it means: A vague, half-formed thought or feeling. https://en.m.wiktionary.org/wiki/henid#:~:text=henid%20(plural%20henids),half%2Dformed%20thought%20or%20feeling.

I agree with Bill, it is a strange word that I can find no reference to in any online dictionary, or the very dusty Collins that I took off my shelf! The closest similar word that might make sense in the context is "Henid" meaning: "A vague, half-formed thought or feeling" That is from Wiktionary though and other references are from blogs and Urban Dictionary - none of which are particularly reliable sources. There's no reference in any major dictionary. It was supposedly coined by the author Otto Weininger. There is no mention of "henidical" though. Perhaps the author took the noun of henid and tried to create an adverb out of it. Like taking the noun "heretic" and creating "heretical". I'm not sure if Jack London is simply making up (or altering) a word. I also think perhaps it might be joke. He's saying that he has this word that no one knows, but actually it's not a real word, so of course no one knows it? As in, maybe he's messing with the reader?

"Henidical" is a very obscure word that I have never seen or heard before. The only online references I can find are other examples of your text from the Jack London novel. The most complete English-language dictionary is the Oxford English Dictionary, which does not have free access online. It is possible to buy a subscription to access it. Some public libraries buy a subscription that library patrons can use. Some public libraries own print editions of this dictionary.

This is rare, but means someone or something that kills hens (female adult chickens). A rooster, for example, could be henicidal.