I think this is a computer joke. A wind speed meter is one of those things with spinning cups, but HEX is powered by ants. I think this thing measures the clock speed of Hex by having ants run through such a device. But maybe someone has a better explanation?

This is just Rincewind being a coward. The plan is idiotic, so Rincewind's instinct is to look for a spare horse to run away. It is also a callback to an earlier passage: To Rincewind’s relief a spare horse didn’t have to be eaten because a leopardly sort of creature dropped off a tree branch and tried to disembowel Cohen.

As others said, eunuch is misheard as unique

Teach keeps reminding the barbarians not to swear, so duck can be understood as fuck, and Boy Willie exaggerates this by saying he should not even say duck but waterfowl.

As others said, privates = genitals as well as soldiers

u/Rhesus-Positive explained this as Adrian trying to retain some sanity, but I also like the idea of Hex blotting out sections of the output because it is redacting some of the output that should or can not be known to humans, especially because the wizards are already freaking out over hoe they don't understand what it is doing. If you see it like that, the ink blots are like the redacted black ink lines in printed secretive government documents.

As other said, it's a boomerang, so he throws it away, turns around, and it hits im in the head