means Drinking in small, deliberate amounts, taking a little at a time rather than swallowing it all at once.

from From the verb "sip," an old English word likely related to "sup" (as in supper) — both rooted in the everyday act of taking liquid into the mouth. It belongs to a small family of words like "sop" and "soup," all gathered around the simple business of drinking and eating. The "-ing" is just the ordinary ending that turns the action into something ongoing — the slow, genteel act caught in the middle of happening.