What is the origin of sesquipedalian?
Sesquipedalian comes directly from the Latin adjective sesquipedālis “having a (linear or square) measure of one and a half (Roman) feet.” Unsurprisingly, sesquipedālis is used in farming, military fortifications, architecture, and construction. The poet Horace (65–8 b.c.) uses the phrase sesquipedālia verba “words a foot and a half long” in his Ars Poetica (c19–18 b.c.), a poem in which Horace sets forth his ideas on “poetic art.” It is from Horace’s phrase that English has its only meaning “having or using very long words.” The first part of sesquipedālis is the adverb and prefix sesqui, sesque “one and a half times,” from an earlier, unrecorded sem(i)que, a contraction of sēmis “one half, a half more” and the generalizing particle –que. Pedālis is easy: it’s an adjective meaning “measuring a foot, a foot long, wide, deep, etc.,” a derivative of the noun pēs (inflectional stem ped-) “foot”; –ālis is a very common adjective suffix in Latin, the source of the English adjective suffix –al. Sesquipedalian entered English in the 17th century.