Absinthe (noun) – a green or sometimes colorless distilled liquor with high alcoholic content that is flavored with wormwood, anise, and other aromatic herbs (such as fennel); also : a similar liquor that is made without wormwood

Source: https://www.merriam-webster.com/dictionary/absinthe

“Yes,” said the girl. “Everything tastes of liquorice. Especially all the things you’ve waited so long for, like absinthe.” (“Hills Like White Elephants”, Hemingway)

I understand the absinthe is an alcoholic beverage which I believe, from what was stated in last class, that since the girl is pregnant, she can not drink absinthe which she probably craves to have. So she wants to wait to have the baby until it will be fine to drink absinthe without affecting the baby. Also, she might me too young to have such a drink. So in that quote, he is referring to her long have or had to wait and comparing it to the wait she had until she was able to have a drink of absinthe.