1º_ It is a mericanism that is used to name a person who is excited by drugs or alcohol, or who looks happy. It is a Castilianization of the English happy ("happy, cheerful"). 2º_ In lunfardo is the vesre de pija ("penis").
It is the way in which we have tried to Spanishize the word happy in English, those who do speak Spanglish since in Spanish we do have the meaning