The original meaning is not universally agreed upon, but the structural composition is not ambiguous. The earliest form of「叚」is comprised of「石」on the left and「又」on the right.

[![enter image description here][1]][1]

For reference, samples of「石」:

[![enter image description here][2]][2]

> Notes:
>
> - 「石」was originally a picture of stone chimes.
> - 「口」was not present originally, being added as a distinguishing component to mark the name of a region.
> - What looks like a cover「厂」in later forms was a graphical reduction of the stone chime shape.
> - The two horizontal marks that appear in some forms are *decorative* (Chinese: 飾筆), and don't contribute to any semantic or phonetic function (decorative marks are also seen in modern forms of many characters like「魚」and「甚」). Later-era *transcribed samples of ancient forms* (Chinese: 傳抄古文字) of「石」which include these marks are:

>  [![enter image description here][3]][3][![enter image description here][4]][4][![enter image description here][5]][5][![enter image description here][6]][6][![enter image description here][7]][7][![enter image description here][8]][8]

Afterwards,「爪」or「刀」was added on the top-right.

[![enter image description here][9]][9][![enter image description here][10]][10]

For reference,「爪」and「刀」were respectively

[![enter image description here][11]][11][![enter image description here][12]][12]

The modern form of「叚」should be seen as composed from「石」(left, inherited from a form with decorative marks and no「口」),「刀」(top right), and「又」(bottom right).

[![enter image description here][13]][13]

---

References:
-

 - 林義光《文源》
 - 何琳儀《說文準聲首輯佚》
 - 季旭昇《說文新證》
 - [小學堂](http://xiaoxue.iis.sinica.edu.tw/)


  [1]: https://i.sstatic.net/nhEaA.png
  [2]: https://i.sstatic.net/F8yYL.png
  [3]: https://i.sstatic.net/SRxxA.png
  [4]: https://i.sstatic.net/f7KPj.png
  [5]: https://i.sstatic.net/mIwWZ.png
  [6]: https://i.sstatic.net/6pPEY.png
  [7]: https://i.sstatic.net/EUtYP.png
  [8]: https://i.sstatic.net/yHvDy.png
  [9]: https://i.sstatic.net/HKrjB.png
  [10]: https://i.sstatic.net/ItxAH.png
  [11]: https://i.sstatic.net/0PF5G.png
  [12]: https://i.sstatic.net/UHmEw.png
  [13]: https://i.sstatic.net/aveAO.png