Hypos: (contemporary usage) The chemical sodium thiosulphate (formerly called hyposulphate) used as a photographic fixer.
Etymology: From the Greek hypo meaning “under.” For example: hypodermic meaning “under the skin” or hypothalamus meaning under the thalamus. Interestingly, hypocrisy (from hypokrinesthai) literally means “under” (hypo) “to sift or decide” (krinein – crisy) and has associations with the stage. An actor would be a hypocrite because he or she is acting a part – saying one thing without necessarily meaning it, playing “under the table.”
Anyways, the word “hypos,” as Melville uses it, is making use of a subtler, psychological definition of the word – Ishmael is, after all, describing his state of mind. He (Ishmael) is talking about “the November in [his] soul,” he finds himself “pausing before coffin warehouses, and bringing up the rear of every funeral” he meets. These sentiments suggest that (a) he thinks he is dying or (b) he is time and time again, through no intention of his own, associating with negative, melancholy circumstances
One fellow suggested in a forum that Ishmael’s “hypos” be replaced with hypochondria. This makes sense, the new a sentence beginning “whenever my hypo[chondria] get[s] such an upper hand of me[…]” However, does this definition hold throughout the entire sentence? How about I write out the whole sentence.
“[…] whenever my hypos get such an upper hand of me, that it requires strong moral principal to prevent me from deliberately stepping out and methodically knocking people’s hats off […]”
Apologies, this is a long sentence and I don’t want to type the whole thing out. The point is this: “hypochondria” as an insert for “hypos” works in the first half of the sentence as well as in the second half. Hypos is making him want to “[step] out and methodically [knock] people’s hats off.”
According to the wondrous Online Etymological Dictionary (a shockingly reputable source), hypochondria did not always mean what it means today. Us 21st century folks use it to mean a false sense of illness – hypochondriacs are those friends who always think they are coming down with some devastating sickness but never actually do and react uproariously to the slightest sniffle or sneeze. However, it meant “depression or melancholy without real cause” back in the 1800s, when good old Melville was writing. Thus, “hypos” might very well mean hypochondria.
Then again, it might actually mean “under.” As in, “under the weather.” This might make more sense than the elaborate explanation above. Ishmael’s already referenced a “November” in his soul – cold dreary weather.
I don’t want to read to much into this other possibility, I believe it is self explanatory. Needless to say, have your dictionaries / thesauruses / and online etymological resources at hand when you’re reading anything as language-dense and archaic as Herman Melville.