much more, still more, the gums
訠 is a rare kanji whose meanings do not share one clear center. One cluster relates to increase or addition ("much more, still more"), while the other refers to the gums. These senses are likely unrelated, so treat them as separate applications.
The exact historical development of 訠 is uncertain. It visually combines 言 (speech) and 引 (pull), but whether this reflects the original meaning or is a later simplification is unclear.
The left side 言 suggests words, and the right side 引 suggests pulling or drawing out. For the "more" sense, imagine pulling more words out of a conversation. For the "gums" sense, picture pulling back the lips to reveal the gums.
For シン, imagine pulling out a shin bone from a conversation about anatomy: shin -> シン, and the words keep pulling out more details.