I suspect you mean 'grasp the nettle'. It comes from the fact that, if you touch a stinging-nettle gently, you'll get stung. However, if you grasp it firmly, crushing its leaf, this does not happen. So, that is the approach you should take to other life-situations...go for it rather than treading carefully around it!
Well, this is what I'm trying to clear up. I think the phrase is grasp the mettle, but over the years it has been misused as grasp the nettle. But maybe I just misheard it when first used years ago!
It comes from one of Aesop's fables, about a small boy crying because he only touched the nettle gently, and his father advising him the "grasp the nettle firmly" to avoid being stung. And Aesop wrote that many hundreds of years ago.
I always thought that it meant, in order to acheive a difficult or painful task (emotionally), you must face up to the pain to carry out the task by doing it and accepting the resulting discomfort.ie; "grasping the nettle". This could be telling a friend some bad news or taking a pet to be put to sleep etc.
Mettle is correct. Nettle is a corruption but of course it sounds more sensible to the ill informed and has therefore achieved ubiquity. A bit like 'the proof is in the pudding'.