What is the Origin of the word "Kosh" ?

As in “to be under the kosh”…

You spelled it wrong: it’s ‘cosh’:

and dictionary.com says: [Perhaps from Romany kosh, stick.]

Not sure about kibosh, though…

At least it isn’t solved as easily as the Kibosh conundrum…

My dictionary is more sure of itself - no “probably” about it:

The Shadows really skewered Kosh.