Used probably due to its resemblance in shape to said baby's toe.
Used in the movie Superbad by Evan when admiring a cover model's tits.
Evan: They're like little babytoes. It's just not fair that they get to flaunt that stuff, you know... and like, I have to hide every erection I get.
Evan: Just imagine if girls weren't weirded out by our boners and stuff, and just like wanted to see them. That's the world I one day want to live in.
Seth: You know what I do? I flip my boner up into my waistband. It hinds it AND it feels awesome. I almost blew a load into my bellybutton.
-quotes taken from imdb.com