1
general::
Phrase(s): get to first base (with someone or something) [and] reach first base (with someone or something)
Fig. to make a major advance with someone or something. • I wish I could get to first base with this business deal. • John adores Sally, but he can’t even reach first base with her. She won’t even speak to him.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, reach first base. 1. 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] 2. 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. [1920s]
American Heritage Idioms