I left some salad leaves out and they went off a bit. My friend said they were "manky" and put them in the recycling.
It's a common word but where did "manky" come from? Looking it up, there's a whole raft of theories, with a lot of interconnections between them, and although there's probably no single path we can trace with certainty, the majority view is that it probably derives from Latin mancus = maimed, which is also the root of French manqué = failed.
Manqué is a strong contender to be the immediate source — we can easily imagine someone pronouncing it "manky" — but it could also have come from cognate words found in other languages, such as Italian mancare or Scots mank.
Wiktionary on the other hand suggests an Old English word *mancian with cognates in other Germanic languages and which could have connections with Latin mancus, and it's even possible all these words come from an ancient Proto-Indo-European root *mank, again with the sense of maimed, damaged, deficient.
It's also been suggested that Polari may have played a part in the word becoming more common from about 1960 onwards. Here's the Google ngram
There are also words such as "mangey" and "mangled" which may be connected with manky in some way, or what have an influence on its use. But in the end it is, as the OED puts it. a word of uncertain origin.
My friend was right about the salad though. It was dead manky.