Where did this meaning of HODL came from? Isn't it that it's just a misspelled typed word of someone who's drunk. Well, whoever and whatever was the meaning of it, why is should be avoided?

You can't tell someone who's been holding for years and then made a decent profit out of it to say that he should avoid it. Experience wise, holding is profitable especially if you're holding good cryptos and the number 1 of it, BTCitcoin.