Method favoured by the Provisional IRA for dealing with informers, drug dealers, or anyone who does something to annoy them or infringe on their territory. Kneecapping is, strangely, most frequently reserved for Catholics, as a non-fatal but nevertheless highly effective deterrent. In this case, their 'territory' refers to run-down, Catholic areas of certain cities in the North of Ireland, Belfast being the best-known example. Generally a drug dealer would be warned at least once or twice by the local IRA before being kneecapped. If he continued to deal drugs after that, not having any knees left to speak of, he would most likely be killed, as I have heard no examples of anyone being "elbowed".
The practice of crippling people by shooting them in the knees had at one stage become so well-known in Ireland that there are widespread jokes about it, including a pun which is utterly unfunny to anyone who is not Irish, relating to the fact that "Ni Ceapaim", the Irish for "I don't think" sounds remarkably similar, when properly pronounced, to "Kneecap him". The full text of one version of this joke is given below for those interested.
Bhi beirt fear IRA ag siul thar an bothar i mBeal Feirste. Chonaic siad fear eile ag seasamh in aice an balla. Duirt fear amhain,
"An ceapann tu go bhfuil an fear seo sa U.V.F.?"
Two IRA men were walking across the road in Belfast, when they saw another man standing next to the wall. The first man said,
"Do you think this guy is in the UVF (Ulster Volunteer Force)?"
"I don't think so."
To say it loses something in the translation is to miss the point of interlinguistic humour completely.