(I'm not a linguist, just generally interested in languages, so forgive me if I lack the appropriate vocabulary.)
There are some examples here, but the general gist is that someone forgets a word (in my case, "shovel") and substitutes a usually multi-word, often inaccurate or outright wrong synonym ("dirt spoon"). This obviously isn't a common synonym for shovel, but gets the point across, especially when there's a shovel visually present to cement the impression. Other examples include "sea pancake" for manta ray and "cereal water" for milk.
There usually seems to be some kind of analogy going on, if not always clear. The speaker tends to default to a similar item and attach an adjective or other descriptor. Not all examples in the link do so, but I'm more interested in the ones that do.
I don't know if this is a frequent linguistic phenomenon or if it's just somewhat prevalent on the internet.