A Lucus a Non Lucendo is an expression meaning to name something after something it is not.
In Latin, a lucus is a dark forest or grove. However, in the eigthteenth century, etymologists who were trying to explain the meaning of the word lucus, came to the conclusion that the word lucus had to be related to the root word lucere, which means to shine. The reasoning went something along the lines of that a lucus would have to be called a lucus because there was no light shining there.
Interestingly, contemporary scholars decided that the explanation was good enough, and for a long time, the reasoning was accepted as fact.
Consequently, a Lucus a Non Lucendo ("A dark grove of no light"), is known as a far-fetched explanation that is very unlikely to be correct, or the act of naming something after something it is not. Examples that have been used is to call a rubber tyre a cuniculus because rabbits do not eat it, to call a mountain an absconditus because it is not hidden, or to call a clown act a maeror because it is not grieving.
Lucus a non lucendo in translation
In translation, a Lucus a non lucendo is basically the act of giving up, after realising that the word does not make sense in the original text: It is the act of having to guess at what the word means - not because it seems to be the correct word, but because it would appear that no other word makes sense in the circumstances.
For example, If someone were to use the Norwegian word "spikerkranie", for example, it could be directly translated with "nail cranium". While it is relatively safe to assume that the "head of the nail" is meant, the word "spikerkranie" does not actually exist in the Norwegian language, and unless the original author would be able to verify the meaning of the word, the translator would have to translate the word with the "head of the nail" (as in "hit the nail on the head").
In extreme cases, the translator would have to add a footnote admitting to have hit a Lucus a non lucendo, and stating that they have inserted the most likely, but not necessarily correct, word into the translation. Normally, the translator would also reproduce the sentence in the original language.