Reminds me of https://en.wikipedia.org/wiki/Rhyming_slang: “The construction of rhyming slang involves replacing a common word with a phrase of two or more words, the last of which rhymes with the original word; then, in almost all cases, omitting, from the end of the phrase, the secondary rhyming word (which is thereafter implied)”
Yep, ‘that’s pony’ means ‘that’s crap’. From pony and trap. You omit the actual word that rhymes.
Nice whistle mate! (I like your suit, from whistle and flute).
It’s fun to figure them out.