Pictogram (象形) of a shield with feather ornaments on top; in some versions in oracle bones, the central and flat part of the shield can be seen as 回.
As an alternative, it's possible 干 and 單 are variant forms of the same character in oracle bone script due to their similar shapes and its appearence in the original forms of 敢. Hence, 干 would depict a bifurcated hunting weapon, not a shield. But 單 has two dots on top that depict two stones, while these two dots are absent in 干.
Also borrowed phonetically as a simplification of 乾 (dry) and 幹. See also 盾 (the pictogram of another shield) and its possible differentiation 古.
The interpretation that links this pictogram to a pestle is folk etymology. Unrelated to 午, which however represents a pestle.
Xíngtiān yǔ dì zhì cǐ (jī gōng zhī guó) zhēng shén, dì duàn qí shǒu, zàng zhī chángyáng zhī shān. Nǎi yǐ rǔ wéi mù, yǐ qí wéi kǒu, cāo gān qī yǐ wǔ.[Pinyin]
When Xingtian fought with the Supreme Deity there (the land of people with odd/uneven upper-arms), the Supreme Deity decapitated him and buried his head at Changyang Mountain. [But Xingtian's] nipples became his eyes, his belly button became his mouth and the thrusting of his shield and battle-axe became his dance.
For pronunciation and definitions of 干 – see 乾 (“dry; moistureless; waterless; arid; to dry; to exhaust; etc.”). (This character is the simplified and variant form of乾).
Notes:
Simplified Chinese is mainly used in Mainland China, Malaysia, and Singapore.
For pronunciation and definitions of 干 – see 豻 (“a kind of wilddog from northernChina, resembling a fox with a blacksnout; etc.”). (This character is a variant form of豻).
For pronunciation and definitions of 干 – see 贛 (“Gan River; Jiangxi, a province of China.; etc.”). (This character is the second-round simplified form of贛).
Notes:
Simplified Chinese is mainly used in Mainland China, Malaysia, and Singapore.
Lục Văn Pảo, Hoàng Tuấn Nam (2003) Hoàng Triều Ân, editor, Từ điển chữ Nôm Tày [A Dictionary of (chữ) Nôm Tày][1] (in Vietnamese), Hanoi: Nhà xuất bản Khoa học Xã hội