ichidan verb
begin to drink; start drinking
Used for starting to drink any beverage, including alcohol. Often implies the onset of a drinking session.
彼はビールを飲み始めた。
He started drinking beer.
のどが渇いたので、水を飲み始めた。
I was thirsty, so I began to drink water.
Compound of 飲み (continuative form of 飲む 'to drink') + 始める 'to begin'.