Fromman +‎baby.

manbaby (pluralmanbabies)

1. (informal,derogatory) Achildishlywhinyman; aman child.
   • 2003, catb...@fwiw.com, “What women want”, inalt.romance (Usenet):

     I happen to enjoy sex very much. (Occasionally a little too much.) But I wouldn't with a snivellingmanbaby who spends all his waking hours, on-line, whining about how life has fucked him over.