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.
For reference, samples of「石」:
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:
Afterwards,「爪」or「刀」was added on the top-right.
For reference,「爪」and「刀」were respectively
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).
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