According to the OED, complementary means “combining in such a way as to enhance or emphasize the qualities of each other or another.” At www.dictionary.com, we find simply “forming a complement; completing,” but then there’s a link to complement. The website gives us three definitions for complement: “1. Something that completes or makes perfect”; “2. The quantity or amount that completes anything”; and “3 .Either of two parts or things needed to complete the whole; counterpart.”
The etymology of complementary tells us that in the 1620s the word meant ceremonious “formally or elaborately polite.” According to www.etymonline.com, the “Sense of ‘forming a complement, mutually completing each other’s deficiencies,’ is attested by 1794, in reference to the calendar of the French Revolution; in reference to colors which in combination produce white light, by 1814.” There was an earlier word complemental, but it is now pretty much obsolete (it has a red, squiggly line under it in my Word document).
Of course, what might be more helpful is the etymology of complement, so here it is from www.etymonline.com: “late 14c., ‘means of completing; that which completes; what is needed to complete or fill up,’ from Old French compliement ‘accomplishment, fulfillment’ (14c., Modern French complément), from Latin complementum ‘that which fills up or completes,’ from complere ‘fill up,’ from com-, here probably as an intensive prefix (see com-), + plere ‘to fill’ (from PIE root *pele- (1) ‘to fill’).” The com- prefix usually means “with,” though sometimes Latin prefixes are used for emphasis, to mean something like completely or thoroughly; if that is the case with complementum, the word means something like “to fill completely.”
Complement should not be confused with compliment “an expression of praise, commendation, or admiration” or “to show kindness or regard for by a gift or other favor.” Interestingly, however, the two words have the same root in the Latin word complementum, though compliment comes into English through the French compliment from “Italian complimento “expression of respect and civility,” from complire “to fill up, finish, suit, compliment,” from Vulgar Latin *complire,” according to www.etymonline.com. Dictionary.com says that it actually came “came to English from the Spanish cumplimiento, by way of Italian and French,” in the 1600s.
So we have two words that are pronounced virtually the same (language nerds like me might make the effort to emphasize the /ɛ/ in complement and the /ɪ/ in compliment) and derive from the same Latin word complementum, but they mean quite different things. So what happened? Well, one of the processes by which languages change is called specialization (or restriction or narrowing), where a word, for some reason or another, means something much more specific than it used to mean. For example, we have an old idiom “meat and drink.” Why “meat and drink”? Why not “food and drink”? Because meat used to mean “food” but has become specialized now to mean a particular kind of food (unless that is your “significant other” telling you to stop eating meat because they want you to starve to death). So when compliment entered the English language, there was already a word filling the slot meaning “to make something complete”; we didn’t need a virtual homophone to mean the same thing, so its meaning was slightly altered.
I was at a hotel some years ago. Above the check-in counter was a sign that read, “Enjoy our complimentary breakfast.” When I said something to the clerk about the sign, he insisted that it was correct. I did not explain to him about my Ph.D. in English. Sadly, the next morning, when I went down to the lobby to get something to eat, the breakfast was very quiet. Perhaps it was just me.
A solitary guy was sitting in a bar when he heard a voice say, “You sure are a handsome looking fellow.” He looked around, but the only other person in the bar was the bartender, down at the other end of the bar cleaning glasses. Then he heard the voice again: “You’re the kind of guy a girl wants to take home to meet her momma.” Again, he couldn’t see anybody.
Then he said to the barkeeper, “Are you talking to me?”
The bartender said no, but the fellow heard the voice again: “I wouldn’t mind taking you home right now.”
Finally, he demanded of the barkeeper an explanation: “Who keeps saying things to me, like I’m good looking, and other things?”
“Oh,” said the bartender, “that must be our complimentary peanuts.”
This image was originally posted to Flickr by vastateparksstaff at https://flickr.com/photos/37922399@N05/16207730152. It was reviewed on 26 May 2016 by FlickreviewR and was confirmed to be licensed under the terms of the cc-by-2.0.
The sad thing is that you can’t hear what the sunset is saying.