The phrase "fruitful language" came up in an answer to the scifi.stackexchange.com question "Was Rian Johnson considered for Episode IX?"
I was not familar with it, nor was another commenter. But the author of the answer left a comment mentioning "it's a common thing rugby commentators say, as someone who is not opposed to the language, yet obliged to apologise as part of their network contract."
I was able to find a few other examples online, confirming that this is not just one person's mishearing or something like that:
Suffice to say some fruitful language came out my mouth after that!!!
(matt-man, Sunday 2nd November 2014; PistonHeads » Gassing Station » TVR » Wedges : "Oh B******s!!")
You could tell from the start that Mark is clearly anti Conservatives and used some very fruitful language towards them, to say the least.
("Live Review: Mark Thomas at Eastleigh’s The Point", By Kieran James, November 12, 2017)
But I could not find a relevant meaning of "fruitful" listed in any dictionary that I checked.
How did "fruitful language" come to have this meaning for some speakers? (Is it related to the use of the word "ripe" to mean "stinking"?) Also, what is the earliest example that can be found of this expression being used?