‘the milk in the coconut’: a puzzling fact or circumstance; alludes to the question of how the milk got into the coconut—of British-English origin (1832), not of American-English origin as stated by the Oxford English Dictionary
USA, Boston Morning Post, 26 May 1834—‘Dutch uncle’: a person giving firm but benevolent advice—unknown origin—perhaps an allusion to Calvinistic sternness
USA, 1930—‘to crawl, or to come, out of the woodwork’: of an unpleasant or unwelcome person or thing, to come out of hiding, to emerge from obscurity; the image is of vermin or insects crawling out of crevices or other hidden places in a building
UK, 1788—denotes an auction in which the price is lowered by stages until a buyer is found—said to have been invented by the Dutch specifically as the best solution to selling tulip bulbs
UK, 1797—strength or confidence gained from drinking alcohol—alludes to the drinking habits ascribed to the Dutch—one of the phrases in which ‘Dutch’ is used derogatorily, largely because of the enmity between the English and the Dutch in the 17th and 18th centuries
After Ambrose Burnside, Union general in the U.S. Civil War, ‘burnsides’ (1866) denotes thick side whiskers worn with a moustache and clean-shaven chin; on the pattern of ‘side whiskers’, it was altered to ‘sideburns’ (1875), itself altered to ‘sideboards’ (1882).
UK, 1953, humorous and euphemistic—‘to fall off (the back of) a lorry’: of goods, ‘to be acquired in dubious or unspecified circumstances’, especially ‘to be stolen’—variant with ‘truck’ came into use later in Australian and North American English
UK, 1844—‘bobby’: a policeman—from ‘Bobby’, pet form of ‘Robert’, in allusion to Robert Peel, who, as Home Secretary, established the Metropolitan Police in 1829—cf. ‘peeler’ (1816), originally a member of the Peace Preservation Force in Ireland established in 1814 by Robert Peel
‘to take the Fifth’: to decline to reveal one’s own secrets—from ‘to take the Fifth Amendment’: to appeal to Article V of the original amendments to the U.S. Constitution, which states that “No person […] shall be compelled in any criminal case to be a witness against himself.”
From ‘rīvus’, ‘a stream’, the Latin adjective ‘rīvālis’ (of, or belonging to, a stream) was used as a plural noun, ‘rīvāles’, to denote persons who have to use the same stream, and persons who have the same mistress, competitors in love.