Liquid courage is a nickname for alcoholic¹ drinks. It is so named because alcoholic drinks seem to have a fortitude-enhancing effect, making you courageous² enough to talk to people you wouldn't otherwise talk to, and not care about things you ordinarily would.

Liquid courage（字面意思为“液体勇气”）是对酒精饮料的一个戏称。之所以有这个戏称是因为酒精饮料似乎有助长勇气的功效，喝了之后能够让人有胆量跟平时不太接近的人搭话，或者变得对平时很在意的事情不那么关注了。

For example:

Adam got some liquid courage up and tried to ask Lisa out, but he was turned down.

亚当喝了几杯酒壮胆之后试图约丽萨出去，可惜被拒绝了。