kɒmplɪˈmɛntərɪ

complimentary: 1) expressive of, or conveying, polite praise or commendation; 2) presented as a gift or gratuity

So a guy’s sitting at the bar, drinking beer, when he hears a voice say, “You’re looking good tonight.” And he looks around, but there’s no-one there. After a while the same voice says, “That new haircut really suits you.” But still there’s no-one there.
The barman, seeing the guy looking around puzzledly, asks him if there’s a problem. And the guy confesses that he keeps hearing a disembodied voice saying nice things about him.
“Ah,” says the barman, “That’ll be the peanuts. They’re complimentary.”

Old joke

The joke plays on the two meanings of complimentary given at the head of this post. The use of compliment to designate an expression of praise or commendation goes back to the seventeenth century. A hundred years later, people began to use the phrase to make a compliment of [some item] when they were describing giving a gift as a mark of praise or respect. That usage of compliment has fallen into disuse, but the second meaning acquired by complimentary lingers on.

Compliment comes (by a roundabout route) from Latin complementum, “that which fills up or completes”, related to the verb complere, “to fill up”, and the adjective completus, “completed”. Complere is the origin of our word complete and the old word complish, “to fill up or complete”, which gives us modern English accomplish. Completus appears (in feminine form) in the Latin phrase completa hora, “finished hour”, which is the origin of the name compline for the final prayer-service of the day in Catholic ritual.

Complementum turned into Spanish cumplimiento, which referred to a very special type of completion—that of fulfilling the complex requirements of formal courtesy. This was adopted into French as compliment, and thence into English. Meanwhile, the idea of meeting an obligation was reflected in the associated Spanish verb cumplir, which passed through Italian and ended up in English as comply.

To further complicate matters, complementum had already found its way more directly into Middle English as complement, “that which completes”. Nowadays, it crops up in specialist uses in grammar, music, optics, mathematics and astonomy. And there’s a set of small blood proteins called the complement system that is activated when our immune system is triggered, helping it complete its destruction of invading microorganisms. The verb to complement means “to make complete”, and things that are complementary serve to complete a whole of some kind—complementary angles sum to ninety degrees, for instance, and complementary colours combine to produce white.

Which brings me, contentiously, to the topic of complementary therapies and the practice of complementary medicine. The underlying claim, evidently, is that these practices somehow complete conventional medical practice by providing something that medicine lacks.* (But at least it’s a better adjective than alternative, which sends the message that you don’t need conventional medicine if you’ve got alternative medicine.)

But we’ve reached the point that refers back to the photograph at the head of this post. It’s a view of the rather pleasingly frosted and lettered window of an establishment quite near my house. (I’ve cropped it down quite severely to remove the establishment’s name, so you’re not seeing it to its best advantage.) There are a couple of things to take issue with in the lettering I’ve shown, but the relevant one for this post is in the top line. I suppose it’s possible that the establishment provides “complimentary therapy”—presumably either free to the recipient or consists solely of flattering remarks—but I’m betting that what was intended was “complementary therapy”. (The error is, of course, extremely common—I once found it in an official hospital communication.)

Latin complere, “to fill up”, is derived from the prefix com-, a marker of intensity, and the adjective plenus, “full, complete”. Plenus has given English a family of words beginning plen- that have something to do with fullness or completeness. To plenish is to fill something up; to replenish is to do that again. Plenty indicates a full supply of something, as does plenitude. Plenary implies that something is full or complete—plenary powers are the most power you can be granted; a plenary session is a meeting at which an entire organization assembles. And an ambassador plenipotentiary has full power to make decisions and take actions on behalf of the administration she represents.

As rare curiosities, I can offer plenilune, “the time of the full moon”, and plenicorn, which designates those ruminants possessing solid horns (like deer) as opposed to those with hollow horns (like cattle) which are cavicorns. A plenisphere is a perfect sphere, and a plenum is a space completely filled with matter—the opposite of a vacuum.

Finally, Latin manus, “hand”, combined with plenus, “full”, to produce manipulus, “handful”. That came into English as the old word maniple, which could be used literally to mean “handful”, or figuratively to mean “small group of soldiers”. And the original meaning of manipulate was “to gather in handfuls”.

And I hope you’ve found something interesting among this latest few handfuls of words.

---

* But then there’s another the old joke that goes: “What do you call complementary medicine that works? Medicine.”