1730, "a trick to 'catch' applause," a stage term; from clap (v.) + trap (n.). Extended sense of "cheap, showy language" is from 1819; hence "nonsense, rubbish."
A CLAP Trap, a name given to the rant and rhimes that dramatick poets, to please the actors, let them go off with; as much as to say, a trap to catch a clap by way of applause from the spectators at a play. [Bailey, "Dictionarium Britannicum," London, 1730]
updated on December 21, 2017