The term comes from a Lenny Bruce routine about African-American men ("Their wangs look like a baby's arm with an apple in its fist" as immortalized in his book _How to Talk Dirty and Influence People_), but this exact wording is taken from The Tubes' song "What Do You Want From Life", from their first album.
Since not everyone gets the reference, the term typically is used as an in-joke.
San Diego slang for a large, flaccid penis. Derived from the similarity in size and curvature to an infant's arm. Also called "baby's arm holding a pear".
"When she unzipped my jeans, I showed her the baby's arm."
The massive boaby of Obid0n, drummer with popular beat combo Subbuteo Pitch Invasion. A man who is so well-endowed and great in bed that when he's done even the neighbours need a smoke.
'Bloody hell, Obi-huge-boaby, that's what I call a baby's arm!'
1st dude - Wots up with you man, your lookin pale and all sweaty.
2nd dude - Just dropped a baby's arm, fukin drained me. Lend us your phone, gotta warn the treatment plant.