take the plunge

verb2

1. •
   To begin any major commitment.

   • And as scientists studying the subject, we will have to conclude that she will take the plunge only if she suffers from overconfidence bias or is innately risk-loving. — 2009
2. •
   To get engaged to be married.

   • She's been seeing William now for about seven months and it looks as if they may take the plunge. — 2001