1923: a small garment worn to cover the genitals—hence (1926): anything intended to conceal something regarded as shameful or indecent—from French ‘cache-sexe’, from ‘cacher’ (to hide) and ‘sexe’ (sex, genitals)
scarce; infrequent; difficult to find or to come by—one early use in 1668—but popularised by the Irish author Thomas Campbell in The Pleasures of Hope (1799)
letter of thanks for hospitality: ‘bread-and-butter letter’ (USA, late 19th century) – ‘Collins’ (UK, early 20th century, from the name of William Collins in Jane Austen’s Pride and Prejudice)