The name reached its all-time peak in 1997 with 12 births ranking #5152. RISING Recent data shows the name is currently rising.
Lennie is a warm vintage short form usually linked with Leonard, so it carries the same broader meaning family tied to lion-like bravery. It feels cheerful and affectionate in modern use.
The name has long existed as a nickname and sometimes stands alone as a first name. Its appeal now is very much part of the revival of soft, informal old-fashioned names.