Everything about "pox leber" as a minced oath, and an earlier source.

[Bluetoes101]

It's funny that this actually works better in English. "I said "goat" not "god", so it doesn't count!".
That's the vibe I get from the below, maybe it is not a good translation (google)?

I'm not sure you would literally write down the words of a false oath? It seems to defeat the point?   
Or is the issue of people literally swearing on false names?

I'm having trouble matching "the scribe is swearing a solemn oath by God's liver." with "swear by vile creatures", "swear by things that do not exist nor will exist", "for no goat has[?] the punishment of punishments"

Is this just a bad translation or am I not following correctly?

"From which it is clear,

first, that those who swear by all words, and falsely by creatures such as "poxplwt samerpox umgel",
thinking that they do not sin, are not bound, ... from what has been said ... are gravely / mistaken.

Secondly, it is clear that those who swear by vile creatures similarly not thinking that they are bound, such as "pox grmt poxlaus / zais",
which is reflected in all these, their creator, are more to be blamed. ... they swear falsely by these, they are perjurers and sin gravely.

Third, it is clear that to this point, those who swear by things that do not exist nor will exist, thinking that they are not bound, such as those who say "sam mir pox gamiger / gamer",
for no goat has[?] the punishment of punishments, nor does God shine in these, whence they seem to sin in two ways,
which they do worse and because they call upon a witness who can testify neither in himself nor in him who shines."