The bit about "From pr [...] indicating contempt" makes me suspect that this is actually a paralinguistic sound, perhaps an ejective bilabial trill. Surely nobody speaking English ever said "pr".

Sounds like a spitting sound.

It reminds me of the sound cartoonists represent as "tsk tsk", which I've heard pronounced "tisk tisk" when think it just means a sarcastic "tch" kind of consonant sound.

Specifically, "tsk tsk" is two consecutive alveolar clicks, as opposed to the two lateral clicks used to get a horse moving.

to get a horse moving
Now I know what tprut reminds me of.
In Russian, "tpru-u-u" is used to stop the horse from moving.

Whoa is me, giddy up, gee up, tis whot i use for me old nag, to stop and to get of its last legs before it goes off to the knackers yard.
Now I be knackered.

Not sure if that's what you mean, but I have always been fascinated by the medieval Czech curse word / ejaculation "krleš". I have encountered this word in various medieval writings, chronicles and such, where it was regarded as a some sort of magical incantation. Some authors would use it in the sense of "woe to me!", some used it as a curse and some even as a substitute for "my God". I was therefore a bit surprised when I found out that it's actually a mutilated version of Greek "kyrie eleison" and it can be traced back to the 10th century Czech version of a litany entitled "Hospodine, pomiluj ny" ("Lord have mercy on us"). Full text: http://cs.wikipedia.org/wiki/Hospodine_pomiluj_ny

Prut is used in modern Dutch, and in various variants in various other Lowlands dialects, to refer to mud, coffee grounds and, by extension, to other filthy and unsuitable stuff. I think it's the sound made by walking through deep mud, in which context the t-initials make sense. Here's more: http://www.surfbirds.com/phorum/read.php?f=56&i=6473&t=6473 I think it turns up in C17th texts, but I can't remember seeing it earlier

Tatyana's "tpru-u-u" is "ptrooo" in Swedish. Bilabial trill?

I'm reminded of 'ptooey'

In order to stop a horse in Danish people use "Pruh" [pru:].

"Prut" [pʁut], though, means "fart". I'm wondering whether it's an onomatopoeticon.

One that has gone out of use in English is "fie". Swedish still has it as "fy".

It's a raspberry, surely?

No but I once heard a commentator on a horse-jumping competition say someone had been "ejaculated" out of his seat!
I take it you're not counting swear words used as exclamations like egad, zounds - although those may not be medieval anyway. How about "fie" and "tush"? I wonder how old the "bah" is in Dickens' "bah humbug". And what did people say before "wow" came into use?

RL Stevenson has this passage in his utterly charming "Travels with a Donkey in the Cevennes," in which he relates his difficulties with Modestine, his donkey:

My deus ex machinâ, before he left me, supplied some excellent, if inhumane, advice; presented me with the switch, which he declared she would feel more tenderly than my cane; and finally taught me the true cry or masonic word of donkey-drivers, ‘Proot!’  All the time, he regarded me with a comical, incredulous air, which was embarrassing to confront; and smiled over my donkey-driving, as I might have smiled over his orthography, or his green tail-coat.  But it was not my turn for the moment.

Another triumph of the Hat Disseminated Intelligence. By adding together small bits of fact such as this one (and the immediately preceding explanation of the "Senyora de la O"), gradually we will attain the elusive Theory of Everything, bit by bit.

I'm waiting for the elusive Senyora de la Tprut.

Before sainthood I believe that her name was Dulcinea, though she had another name before that one which I forget.

IIRC Michaux wrote a poem about Le Maitre d'O,

-It's a pity this savory ejaculation has fallen out of use. Anybody have other examples from medieval languages?-

Conmpletely agree. Isn't amazing how we have become more and more intelligent and less and less colorful.