Regardless of whether you want to talk about "etymology" or "glyph origin", the first step is to find the earliest appearances of the character and try to gauge what kind of word or words it represented. Bag is not the earliest meaning of 「包」; the word represented by this character should be interpreted as either
- to bundle, to wrap (verb). From the Classic of Poetry:
In the wild there is a dead antelope,
And it is wrapped up with the white grass.
- a bundled/wrapped object (noun):
I order you to oversee and govern the officials. If bribery occurs, chaos will ensue.
Note: "Bribery" is a metaphorical translation of 「包」, which should be interpreted as wrapped gifts in this context.
Baxter-Sagart (2014)'s reconstructions and explanations may offer some insight:
- 「包」 /*pˤ<r>u/ (to wrap, to bundle)
- 「抱」 /*[m-p]ˤuʔ/ (to carry in the arms)
Here, the dash "-" indicates a morpheme boundary. The authors list several affixation categories, including an *m- prefix and *<r> infix, which may both occur in related words, e.g.
- 「挾」 /*m-kˤep/, to grasp
- 「夾」 /*kˤ<r>ep/, to press between
From this, I would consider it quite likely that 「包」 shares etymology with 「抱」.