To begin well; take a successful first step •Usually in the negative: I couldn't get to first base with the committee(1930s+)
To initiate sexual activity successfully, esp by hugging, caressing, kissing, etc •In the same baseball analogy, get to third base means touching and toying with the genitals, and get to home plate means to do the sex act, that is, ''score'' (1970+ Teenagers)
In early sexual play, holding hands: ''In sixth grade,'' she said, ''first base is holding hands, and second base is kissing on the mouth''(1980s+ Schoolchildren)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with get to first base
reach first base.
Succeed in the initial phase of something; meet with preliminary approval. For example, They were delighted that they'd gotten to first base in the negotiations. This term alludes to the first base of baseball, which is the first step toward scoring a run for the batter's team.
[ c. 1900
Reach the initial stage of sexual intimacy, such as kissing. For example, Mary is so shy that I can't even get to first base with her.