What is the X in X-mas?
1836, kye-bosk, in slang phrase put the kibosh on, of unknown origin, despite intense speculation. The earliest citation is in Dickens. Looks Yiddish, but origin in early 19c. English slang seems to argue against this. One candidate is Irish caip bháis, caipín báis "cap of death," sometimes said to be the black cap a judge would don when pronouncing a death sentence, but in other sources identified as a gruesome method of execution "employed by Brit. forces against 1798 insurgents" [Bernard Share, "Slanguage, A Dictionary of Irish Slang"]. Or it may somehow be connected with Turkish bosh (see bosh).