Wiktionary
n. (context idiomatic English) A relatively weak (low alcoholic content per volume) and voluminous mixed drink, typically containing lots of ice and mixer.
Wikipedia
A long drink or tall drink is an alcoholic mixed drink with a relatively large volume (> 12 cl, frequently 16–40 cl or between 5–9 fluid ounces).
A long drink will have a tall glass full of mixer, in contrast to a short drink which has less mixer. Short drinks are generally stronger since both types tend to contain the same amount of alcohol. Long drinks are therefore generally more dilute than short drinks.
A classic long drink is a Tom Collins. A simple style of long drink is the highball, a cocktail composed of one liquor and one mixer, (excluding garnish or ice). A classic example of the highball is the gin and tonic.
Usage examples of "long drink".
I took a good, long drink of ale and he finished his Scotch and water.
Taking a long drink from his cup, Robert looked down at the message on the beeper.
He took a long drink and stared moodily into the cloudy mirror behind the bar.
Ladirno took a long drink of his wine thinking of ducal favor .
He took a long drink, grunted with pleasure, and looked around him.
When we get back to my office you'll get a nice long drink of warm milk before sleepy time.
He uncorked it and took a long drink, then gave it a long look before shaking his head once again, then absentmindedly recorking it and putting it away without offering Niko a drink.