late 14c., "represent in visual arts; put into words," from Old French espresser, expresser "press, squeeze out; speak one's mind" (Modern French exprimer), Medieval Latin expressare, frequentative of Latin exprimere "represent, describe, portray, imitate, translate," literally "to press out" (source also of Italian espresso); the sense evolution here perhaps is via an intermediary sense such as "clay, etc., that under pressure takes the form of an image," from ex "out" (see ex-) + pressare "to press, push," from Latin premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike"). Related: Expressed; expresses; expressing; expressible.
early 15c., expressioun, "action of pressing out;" later "action of manifesting a feeling;" "a putting into words" (mid-15c.); from Late Latin expressionem (nominative expressio) "expression, vividness," in classical Latin "a pressing out, a projection," noun of action from past-participle stem of exprimere "represent, describe," literally "press out" (see express (v.)). Meaning "an action or creation that expresses feelings" is from 1620s. Of the face, from 1774. Occasionally the word also was used literally, for "the action of squeezing out." Related: Expressional.
I have retain'd the word BREECHES, as they are known by no other name amongst country folk.--The change from vulgarity to refinement, in cities and towns, has introduced other appellations; there they are generally called SMALL CLOTHES, but some ladies of high rank and extreme delicacy call them INEXPRESSIBLES. [footnote in "Poems Miscellaneous and Humorous," by Edward Nairne, Canterbury, 1791]
Inexpressibles is the earliest recorded and thus seems to have begotten the trend: Unmentionables (1806); indispensibles (1820); ineffables (1823); unutterables (1826); innominables (1827); and inexplicables (1829) followed.