sip at

sip at (something)

To take small sips of a drink over a period of time. My favorite thing to do is watch the sun come up while I sip at my first coffee of the day. It always makes me self-conscious when someone I'm out with sips at one or two drink throughout the whole evening while I'm getting a fresh pint every half hour.
See also: sip

sip at

or sip on
v.
To drink from some beverage in small amounts at a time: The skier sipped at the hot coffee at the lodge. She was sipping on a cocktail at the bar.
See also: sip
References in classic literature ?
There was an attractive spot among the trees where were a great many wooden tables and benches; and there one could sit in the shade and pretend to sip at his foamy beaker of beer while he inspected the crowd.