An oath.
I.e., "look at that slaughtered pheasant!" / "deem me equal to that slaughtered pheasant!"
Meaning: "If any of us shall violate (the oath of never going back to Wey), we will meet the same fate as that of the sacrificial pheasant."
The glossing of 昧 (OC *mɯːds) > 昧 (MC mwojH) as 割, and reading as 刎 (MC mjunX) < 刎 (OC *mɯnʔ), is problematic. See Chen Li: 《公羊義疏》 https://archive.org/details/02074773.cn/page/n8/mode/1up
Consider the possibility that 雉 might be a typo fixated in the textual tradition.
It might be that splitting is not necessary, as 雉 might has well been a typo for 維, and the reconstructed sentence becomes even more laconic (昧,維彼視).
Chen Li himself considers 昧 (OC *mɯːds) to be a typo of 昩 (OC *maːd), and connects it to 抹 (OC *maːd) which has the meaning of 刎 (OC *mɯnʔ), 伆 (OC *mɯnʔ, *mɯd), and 歾 (OC *mˁut) . However, the usage of 抹 (OC *maːd) as "to kill, to cut, etc." seems a bit modern.
See external link. The problem is that this usage, if supportable, appears rather rare.
Intriguingly, the Guoyu wrote of Prince Shensheng's death as 雉經于新城之廟 (page). The Eastern-Han-era dictionary Shiming says 雉經 means dying from asphyxiation with bent neck "in the manner how a pheasant does it". The pheasant in classical culture has a reputation of never submitting to live capture; it always dies fighting or by suicide. However, it is questionable whether it is possible at all to die from suicide in this manner by bending one's own neck. The Qing-era commentary on the Shiming quotes (page) from Zheng Xuan's commentary to the Liji and subsequent commentary by Kong Yingda et al. (page), by linking 雉 to the rare character 絼, rope for pulling a cattle by the nose. That character could be a variant of 紖 (OC *l'inʔ), possibly related to 雉 (OC *l'iʔ) by pronunciation. This is corroborated by the Zuozhuan where Shensheng's death is said to be from self-hanging. It's unclear whether this meaning and its context could be related to 昧雉彼視 via 雉.
This weird phrase found in excavated stone tablets, interpreted as the conclusion of oaths, has been linked to the phrase here.
Some reading based on near homophony was proposed, such as "滅 (OC *med)夷 (OC *lil)彼 (OC *pralʔ)氏 (OC *ɡjeʔ)". However this instance of near homophony is more likely in modern Chinese than in OC.
Perhaps a closer homophone would be
I.e.
Or alternatively, a straightforward reading without homophone characters: "Destroy (夷, to flatten) like reaping hemp plants (麻) not like this.". Cf.: 殺人如麻/杀人如麻 (shārénrúmá).