Where does the word “sesquipedalian” come from and What does sesquipedalian mean?

It was the Roman poet Horace who coined the very long adjective “sesquipedalian” that is used to define a word that is very long.

It appears in line 97 of one of the last of his works, published shortly before his death in 8 B.C., the Epistle to the Pisos, better known as Ars Poetica (The Poetic Art):

Et tragicus plerumque dolex sermone pedestri
Telephus et Peleus, cum pauper et excul, uterque
Projicit ampullas, et sesquipedalia verba,
Si curat cor spectantis tetigisse querela

We are indebted to Miss Dorothy Gardner for assistance with the translation, of which the following is a free rendition into verse:

By use of ponderous speech to tell each tragic part
Did Telephus, the pauper, and Peleus, outcast,
Attempt to touch with grief the watcher’s kindred heart,
Emitting yard-long words and spouting forth bombast.

Literally, sesquipedalian means “a-foot-and-a-half long,” from sesqui-, “one-and-one-half,” and pedalis, “foot-long,” from pes, “a foot.” Figuratively, and it is in this sense that Horace coined it, the word is entirely equivalent to our yard-long, block-long, mile-long, etc., which we use to express an indeterminate length that is substantially more than is necessary, or than is expected. Miss Gardner also points out to me that the Greeks anticipated us here, too, with terms such as amaxiaia remata, “words large enough for a wagon.”