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The expression tennis elbow designates a form of repetitive strain injury that affects the tendons of forearm muscles attached to the lateral epicondyle of the humerus.—Synonym: lateral epicondylitis.

—Note: Although first identified in tennis players, this type of inflammation is not necessarily caused by playing tennis. The following, for example, is from Greg Schiano, GM? The coach is relying on NFL experience in a changing sport, a portrait of the general manager of the Rutgers football program, by Steve Politi, published in The Star-Ledger (Newark, New Jersey, USA) of Wednesday 26^th March 2025 [page E6, column 1]:

Gap-tooth smile. Prominent chin. Tennis elbow from years of chopping wood.
Introducing GM… Greg Schiano!

The earliest mention of this type of inflammation that I have found is from the following letter, published in The Field, The Farm, The Garden, The Country Gentleman’s Newspaper (London, England) of Saturday 14^th May 1881 [page 668, column 2]:

ARM STRAINED BY LAWN TENNIS.—I write on a subject which is, I believe, of moment to many lawn tennis players. For some years I have been a constant player, and during the last twelve or eighteen months have been suffering more or less from a strained arm. As another member of my club has been affected in precisely the same way, it seems as if such a strain is a not uncommon incident of the game, and that as we have a clergyman’s throat and a housemaid’s knee, we may come to hear of a tennis player’s arm, and I therefore append a few particulars, hoping thereby to elicit from some of your readers suggestions as to prevention and cure. The strain is felt in the muscles of the thick part of the fore arm, rendering it more or less useless for the purposes of gripping any weight with the fingers and raising it at arm’s length: e.g., I found it very painful, when my arm was at the worst, to raise a teapot at arm’s length. The pain is especially localised in a small bone just below the elbow, which is tender to the touch. As to cause, my explanation is that when the bat is tightly gripped, the muscles connected with the fingers are rendered extremely rigid, as may be seen by the way in which they stand out. When in this rigid state, the bat exercises upon them, in certain strokes, a very powerful leverage, and hence comes the strain. The most trying stroke is the over-hand service; the bat is usually held at the extreme end, much force is used, and the tendency is to suddenly arrest the momentum of the bat as soon as the ball has been struck. It is, I believe, this sudden arresting of the stroke that does the mischief, as its unexpended force converts the bat into a long and powerful lever, acting on the rigid muscles of the wrist and fore arm. I do not know how far it is possible to allow the bat to swing free like a pendulum or a croquet mallet, and shall be glad if any experienced player can inform me. It is possible that to arrest the bat in the above manner is an awkward trick, which a skilful player would avoid. I have tried rubbing my arm with brandy, usually very effective with strains, but in this case with no result.—R. C. P. [We should advise our correspondent to rub in daily the following liniment—viz.: Liquid ammonia, laudanum, spirit of turpentine, and soap liniment, of each half an ounce.—Ed.]

The earliest occurrence of the expression tennis elbow that I have found is from a reply to the above-quoted letter, published in The Field, The Farm, The Garden, The Country Gentleman’s Newspaper (London, England) of Saturday 21^st May 1881 [page 701, column 1]:
—Note: verbum sap is short for the Latin phrase verbum sapientī (sat est), i.e., a word to the wise (is sufficient):

TENNIS-ELBOW.—As your correspondent C. R. P. [sic] surmises, this is a well-known complaint among tennis players. It to due to over-use of the elbow joint and parts around it. The treatment is simplicity itself, and may be summed in one word—rest. For the time lawn-tennis must be given up altogether, unless the patient possesses the determination of the amateur tennis (not lawn-tennis) champion, who, when suffering from tennis-elbow in his right arm, taught himself to play, and pretty well too, with his left. As tennis-elbow usually occurs in young and healthy subjects, who take plenty of exercise, it generally rights itself in a short time. If much inflamed and painful, soothing applications, such as Goulard water, may be used; if not acute in character, a stimulating liniment, such as given in The Field last week, may be used. In a non-acute case (I avoid the word “chronic,” as it rarely becomes chronic), a friend of mine derived, or thought he derived, great benefit from Whitehead’s essence of mustard. But I suspect that one stimulating liniment is about as useful or useless as another, so long as it serves to amuse the patient. The remedy is rest; and, verbum sap, don’t play tricks with the arm when it is getting well. Don’t say, “My arm is nearly well. I think I might have a game to-day, if I play quietly.” But stick to resting it for some time after all symptoms of uneasiness have entirely disappeared.—H. J.