Going out to drink — drinking alcohol outside the home (at izakaya, bars, outdoors) with friends or colleagues; colloquial and opposite of 'drinking at home'.
金曜は同僚と外飲みの予定がある。
I have plans to go out drinking with colleagues on Friday.
最近は外飲みより家飲みのほうが増えた。
Lately drinking at home has become more common than going out to drink.
外飲みだと帰りが遅くなりがちだから注意してね。
Be careful — going out to drink tends to make you come home late.