One of the more interesting extinct first-person pronouns of Japanese is shizu. Spelt 賤, meaning "base," or "lowly," this was popular among rakes and wags during the Edo period but is now restricted to certain professions carrying on Edo-period traditions (notably, entertainment).

Before this, it was an adjectival noun with the meanings, well, "base" or "lowly", as seen in this example from the "Yūgao" chapter of the Genji, where Genji lives like common people and does whatever common people do:

八月十五夜、隈なき月影、隙多かる板屋、残りなく漏りて来て、見慣らひたまはぬ住ひのさまも珍しきに、暁近くなりにけるなるべし、隣の家々、あやしき賤の男の声々、目覚まして、「あはれ、いと寒しや」、「今年こそ、なりはひにも頼むところすくなく、田舎の通ひも思ひかけねば、いと心細けれ。北殿こそ、聞きたまふや」など、言ひ交はすも聞こゆ。

Which Suematsu "First English translator of Genji" Kenchō 末松謙澄 renders:

It was on the evening of the 15th of August when they [Genji and Yūgao] were together. The moonlight streamed through the crevices of the broken wall. To Genji such a scene was novel and peculiar. The dawn at length began to break, and from the surrounding houses the voices of the farmers might be heard talking.

One remarked, "How cool it is." Another, "There is not much hope for our crops this year." "My carrying business I do not expect to answer," responded the first speaker. "But are our neighbors listening!"

Well, he divides the dialog up differently, and I hope I haven't offended any farmers (typical viscount), but I think you get the idea.

Anyway, this shizu is commonly etymologically broken down to a /si/ meaning "lower" (thus related to shita/shimo 下, and maybe shizumu 沈む, etc.) and a voiced version of the /tu/ roughly corresponding to modern no in words like umitsuji 海つ路 "sea route."

So one interesting question is whether it is also related to the word shizuka 静か, which in modern usage means "quiet" or "still." /ka/ is just an adjective-forming affix, so morphologically it could be possible. Most sources seem fairly confident in linking /siduka/ to /sidumu/, and /sidumu/ to /sita/, and /sita/ to /sidu/, but there doesn't seem to be enough evidence to declare them all part of the same word-family without any reservations. (Although Orikuchi Shinobu 折口信夫 gave it a go. He argues that the root meaning is "sink," with "lower" being derived later.)

Anyway, if we can relate shizu 賤 to shizuka 静か and shita 下, we would have four words deriving from the same roots but meaning different things:

• shizugokoro 静心 = "still heart/soul" (e.g.)
• shizugokoro 賤心 = "base/lowly heart/soul"
• shitagokoro 下心 = originally "secret heart/feelings" (e.g. in MYS 1308: "komoritaru/ a ga shitagokoro/ ko no ha shiru ramu" = "the leaves on the trees know the secret thoughts I hide"), later "secret intentions" or "ulterior motive"
• shitagokoro 下心 = "lower heart": the radical at the bottom of kanji like 忘 and 悲 (okay, this one is kind of cheating)

Interestingly, in the essay I linked above, Orikuchi claims that the instances of 下心 in the Man'yōshū should be read read shizugokoro, but I can't find this reading in any modern edition I checked, so I'm not sure if this is a now-abandoned older reading or just Orikuchi being idiosyncratic (he notes even in the essay that others disagree).