Green’s Dictionary of Slang

first base n.

[baseball jargon]
[1920s+] (orig. US)

1. a man’s initial advances on a young woman, usu. implying just kissing, but sometimes also caressing some part of the body or even the removal of some clothing; such a ‘base’ is always above the waist; thus second base under second adj.; third base under third adj.; home run n.

2. the start of something, e.g. a relationship, usu. applied in terms of failure, e.g. he couldn’t even get to first base.