foot
呎 is a rare kanji used specifically for the unit of length 'foot' (as in feet and inches). Its meaning is narrow and technical, with no broader conceptual center beyond this measurement.
呎 is a modern Japanese coinage combining 口 (mouth) and 尺 (shaku, a traditional unit of length) to represent the English word 'foot' as a unit of measurement. The 口 likely serves as a semantic marker for a borrowed word or unit, while 尺 provides the measurement context.
The left side 口 (mouth) can be thought of as a 'foot' (the body part) opening wide, while the right side 尺 (shaku) is a ruler. Together, they measure a foot in length.
For シャク, imagine a shaku ruler (尺, しゃく) measuring exactly one foot: the reading シャク comes directly from 尺, which is already a unit of length.