"The cold pierces you to the marrow."
Translation:Tá an fuacht tríot.
This is a very irritating question that must be answered to get through the lesson. It is, at best, an idiomatic phrase in English. If it must be used then the English translation should closely mirror the Irish...something like The cold "goes" through me. If the Irish tree wants to play with idioms and very idiomatic phrases they should put this in a special section that can be bought with lingots, like in Spanish, and then have fun with it for learners that want to memorize them.
There are much better ways to teach/drill the preposition tri than with a trick question.
In this case, you don't have to answer with the idiomatic translation to get this question correct. I never do because it would be unnatural for me in my native dialect, so I understand your frustration. If you answer, "The cold is through you," Duolingo will accept it as correct. The biggest trick is to pay attention to whether the sentence is presented as "Tá an fuacht tríot," or "Téann an fuacht tríot." The distinction between "The cold is through you," and "The cold goes through you," does matter. Watch that verb!
Duolingo always gives me this question as a select-the-word-order question, so I can't use "the cold is through you" (I like literal translations because they help me see how the sentence goes together). It only has the verb "pierces", and "The cold pierces you" isn't accepted without "to the marrow"
"Téann an bhfuacht isteach i do chnámha" is a phrase Irish people really use, at least in my part of the country, but isn't accepted unfortunately.
The real icing on the cake for this exercise is that the expected answer given above is directed to a single person ("tríot," second person singular), but the Dictionary Hint when you hover over the word "you" ("sibh," second person plural) implies the Duo is expecting an answer directed at multiple people. Given how little the actual answer resembles the translation prompt and the inconsistency with other exercises ("Tá an fuacht tríot" vs "Téann an fuacht tríom") it genuinely feels like this specific exercise was intentionally designed to make people fail.
I think the reference is to hovering over words when you are doing the exercise (when the exercise gives the Irish to be translated to English).
You can also look in the Duolingo dictionary from the main menu (under More) or if you look at a sentence discussion for the Irish version of a sentence the words are often links to the dictionary entry. For example, see the https://forum.duolingo.com/comment/4617190
On a computer with a mouse, you hover by placing the cursor on the word without clicking. On a touchscreen device like a phone or tablet you can't hover, but you can access the Duolingo dictionary for a word (if the word is blue) by tapping the word. This isn't the same information as shown when hovering, though.