Green’s Dictionary of Slang

first base n.

[baseball jargon]
(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.

[US]J. Lait Broadway Melody 75: With all his jack I don’t think he’d get to first base with Queenie.
[US](con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 147: He liked Cabby Devlin, but he couldn’t get to first base with her.
[US]R.E. Sherwood Idiot’s Delight 148: Did you get anywheres near to first base, Harry?
[US]J.D. Salinger Catcher in the Rye (1958) 84: I knew she wouldn’t let him get to first base with her.
[UK]G. Lambert Inside Daisy Clover (1966) 28: There’s a couple of them I let take me to the movies, but they’ll never make first base.
[US]S. Longstreet Straw Boss (1979) 273: I’ve been married twice, both times tossed out on first base.
[US]G.A. Fine With the Boys 169: First base, n. (1) Kissing [...] (2) Lips.
[US]D. Burke Street Talk 2 170: I got to first base with her last night.
[UK](con. 1981) A. Wheatle East of Acre Lane 78: Talking of being a loser [...] you get to first base wid Carol?
[UK]A. Wheatle Crongton Knights 20: ‘I’m not into that first-base friend-zone crap’.

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.

[US]J. Lait Put on the Spot 112: They won’t get to first base.
[US]J.T. Farrell ‘A Sunday in April’ in To Whom It May Concern 150: The boys at the Hall often try, but they don’t get to first base.
[US]‘Hal Ellson’ Tomboy (1952) 100: You’re jealous and can’t get to first base with Lucky.
[US]R. Prather Scrambled Yeggs 78: Four strikes I’d had at the guy, once on the phone, three times inside and I still hadn’t made it to first base.
[UK](con. 1981) A. Wheatle East of Acre Lane 57: Me an’ you won’t even get to first base if you’re still carrying on wid dem t’ings der.
[UK]A. Wheatle Dirty South 125: If you wanna get to first base with them bitches you have to chat about what kinda schools will your children go to.