1607, as ‘as snug as pigs in pease-straw’—especially in ‘(as) snug as a bug in a rug’, phrases built on the pattern ‘(as) snug as [animal name] (in —)’ mean ‘in an extremely comfortable position or situation’
meaning: ‘what is appropriate in one case is also appropriate in the other case in question’—earliest occurrence in A Collection of English Proverbs (1670), by John Ray
Irish English, 1829—‘in a fortunate or prosperous state’—a loan translation from Irish ‘ar mhuin na muice’, meaning, literally, ‘on the pig’s back’, and, figuratively, ‘in a fortunate or prosperous state’
USA, 1986—something intended to divert attention from something more important—refers to the fact that when two male gorillas confront each other, they throw dust in the air to distract one another—popularised, if not coined, by Henry Ross Perot
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