UK, 1921—‘frogspawn’: a jocular appellation for ‘tapioca pudding’ (also for ‘sago pudding’)—originated in schoolchildren’s slang—refers to the fact that both tapioca pudding and sago pudding very much resemble frogspawn, i.e., a soft substance like jelly which contains the eggs of a frog
Australia, 1937—very scarce—‘rocking horse’: a toy horse mounted on rockers or springs for a child to sit on and rock to and fro—this phrase has come to be also used in British English and American English
Australia, 1982—a coded signal from a man to other men, indicating that a woman is approaching, so that they all moderate their language—originally used in shearing sheds, but now used in other places, especially in pubs
Australia, 1930—a humorous extended form of ‘flat out’, meaning ‘with the maximum speed or effort’ (apparently with wordplay on ‘flat out’, meaning ‘lying stretched out’)—has occasionally been used in the opposite sense
Australia, 1972—a person, usually a man, who, in spite of a superficial smartness, is untrustworthy—‘rat’ refers to a deceitful or disloyal man—the image is that, despite the gold tooth, a rat’s basic nature cannot change
Australia, 1928—an outstandingly good person or thing—variant of the synonymous jocular expressions, of U.S. origin, based on various parts of animals’ real or fanciful anatomy and other attributes, such as ‘the bee’s knees’ and ‘the cat’s whiskers’
USA 1907—used of the inanity of life and of the transitoriness of success—originated in the captions to a cartoon known as ‘The Dejected Rooster’, by Mark Fenderson, published by the weekly magazine Life (New York City) in 1907
USA, 1893—utterly despicable—jocular extension of ‘lower than a snake’—refers to the use of ‘low’ to mean ‘despicable’, and to the use of ‘snake’ to denote ‘a treacherous or deceitful person’
USA, 1982—a catchy song or melody that keeps repeating in one’s mind, especially to the point of irritation—loan translation from German ‘Ohrwurm’—original meaning (1598): an earwig