The law lords of the House of Lords were formally known as the lords of appeal "in ordinary". Most of that phrase other than the final bit is self explanatory but the use of the word "ordinary" just seems quite odd. What is its intended meaning?
Between 1876 and 2009, there were three ways to be a "Lord of Appeal", a member of the House of Lords who was part of its committee exercising judicial functions. (Before 1948 it was not a formal committee; proceedings took place in the Lords chamber as part of the House's normal business.)
- You could be Lord Chancellor. The most recent LC to have done this was Derry Irvine, Lord Irvine of Lairg (e.g. in Boddington v British Transport Police  UKHL 13).
- You could be a regular member of the House of Lords who was qualified to sit as a judge, by virtue of having held "high judicial office". In principle, this would include hereditary peers who had taken a legal career path, but most often meant life peers. That would include distinguished members of the judiciary who had been given a life peerage, former holders of the office of Lord Chancellor. and (since 1889) retired Lords of Appeal in Ordinary. For example, Lord Mackay of Clashfern (LC until 1997) continued to hear cases for several years on an ad-hoc basis. There were also current former Masters of the Rolls, Lords Chief Justice, and senior Scottish judges in this category.
- You could be appointed as a "Lord of Appeal in Ordinary", which means that you were named to the House of Lords in order to serve as a judge (and by convention, not to participate in debates and votes). The "in Ordinary" phrase expresses that you are a Lord of Appeal as your main job - not because it's an additional job on top of being a peer.
Before the reforms of the late 19th century, any peer could participate in judicial business, at the discretion of the House in general. Some peers with legal experience were de-facto regarded as "law lords", and were the ones who by consensus took charge of appeal cases. The last time a lord without this experience tried to do this was 1883, in Bradlaugh v Clarke: Lord Denman (whose father had been Lord Chief Justice but who himself had no legal qualifications) tried to vote but the Lord Chancellor ignored him. That follows a precedent of 1844 (O'Connell v The Queen) that other Lords should gracefully decline to contribute, once the Law Lords had given their opinions.
Even in that century, the emerging and eventually formalized category of "law lords" did not capture the nuance of who was making judicial decisions. The committee was typically "assisted" by professional judges, drawn from the serving judiciary, who would privately give their opinions to the actual named Lords. This was also the system used in trials of peers, where the House sat as a court of first instance if a peer was charged with a felony: nominally, any lord could take part in proceedings, but in practice the process would be stage-managed by a qualified Lord, with a panel of normal judges quietly doing much of the real work.
In relation particularly to the staff of the British Royal Household, and more generally to those employed by the Crown, it is used as a suffix showing that the appointment is to the regular staff, for example a priest or chaplain-in-ordinary, or a physician-in-ordinary, being a cleric or doctor in regular attendance. The usage goes back to the 17th century. See for example:
- Principal Painter in Ordinary
- Lords of Appeal in Ordinary
- Chaplain Extraordinary