1. Pathetic argument

Pathetic as evaluative
A participant can dismiss an argument she utterly rejects as “a pathetic, pitiful argument” because childish, void or desperate. One can say “I find this argument pathetic” (evaluation), but not “I find this argument a pari”, only “In my view, this is an argument a pari” (description).
The label pathetic argument is evaluative and can be applied to any kind of argument scheme.

Pathetic as descriptive
The label “pathetic argument” can be descriptively applied to a variety of arguments based on negative or positive consequences. The conclusion is deemed impossible and rejected because it would frustrate the arguer; or taken for granted because agreeable to her.

I fear that P, so not-P.
I wish P, so P
It can’t rain on Sunday, our picnic would be ruined!
This is not possible, we couldn’t manage the consequences:

— Syldavia cannot suspend its payment, that is impossible, nobody knows what might happen, actually we wouldn’t know how to deal with such a situation.
— Such pollution is unthinkable, it would make thousands of victims.
— If this criticism were right, what would become of our discipline?

The pathetic argument applies to the field of knowledge a style of argument quite common in the field of practical action:

I wish that P, so I strive to achieve P, I pray for P, I try to bring about P.
I fear P, so I try to avoid, prevent P…

But wishing P is different from striving to achieve P. That kind of  argument can systematically be evaluated as pathetic that is, “naive and desperate”.

The pathetic argument is currently designated as the argument ad consequentiam or the appeal to consequences. Here, the pathetic argument is considered as just one particular kind of appeal to consequences.

Pathetic arguments are not pathemic arguments. Pathemic is a derivative from pathos; one can speak of a pathemic arguments to refer to ad passiones arguments, that is, to any emotion-based argument, such as appeal to anger, enthusiasm, pity, etc.
Pathetic, “pitiful”, it can only be considered as a sub-sort of the pathemic argument.

2. “Pathetic fallacy”

The label “pathetic fallacy” refers to the anthroTpomorphic attribution of human feelings to non-human, non-living beings; it condemns the use of the rhetorical figure of personification. The expression was coined by John Ruskin:

I want to examine the nature of the other error, that which the mind admits when affected strongly by emotion. Thus, for instance, in Alton Locke,
They rowed her in across the rolling foam
The cruel, crawling foam.
The foam is not cruel, neither does it crawl. The state of mind which attributes to it these characters of a living creature is one in which the reason is unhinged by grief. All violent feelings have the same effect. They produce in us a falseness in all our impressions of external things, which I would generally characterize as the ‘pathetic fallacy’.
John Ruskin, Of the pathetic fallacy, [1856][1]

The label pathetic argument is consonant with the label pathetic fallacy. The pathetic fallacy condemns the personnification of natural world, while the pathetic argument seems to consider that the natural world is subject to the human desires.Both moves blurs the boundaries between the human and the natural world.

[1] In Modern Painters, vol. III, part IV, London: Smith Elder, p. 160. Alton Locke is a novel by Charles Kingsley (1850).