a shoe, the sole of a shoe, magpie
舄 is a rare kanji with two distinct meanings: a shoe or its sole, and the magpie bird. These meanings do not share one clear conceptual center, so treat them as separate applications.
The exact historical development of 舄 is uncertain. It may have originated as a pictograph of a bird or a shoe, but the modern form does not clearly reflect either origin.
舄 looks like a mortar 臼 sitting on a base with a wrapping enclosure 勹 and fire 灬. Imagine a mortar-shaped shoe sole being heated over a fire to shape it, or a magpie perched on a similar rounded nest shape.
For シャク, picture a magpie wearing a tiny shoe that makes a 'shak' sound as it hops: shak -> シャク.