An infant, as in She's been a family friend since I was a babe in arms. Although the word “babe” for baby has been used since the 1300s, this phrase describing a child too young to walk (and hence having to be carried) dates only from about 1900.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use babe in arms in a sentence
There were a half dozen of them—two men, three females, and one babe-in-arms.Quest of the Golden Ape | Ivar Jorgensen