From The Collaborative International Dictionary of English v.0.48 [gcide]:
moo-cow \moo-cow\ n.
a cow[1]; -- a child's word. See 1st cow[1], n.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) [wn]:
moo-cow
n 1: female of domestic cattle: "`moo-cow' is a child's term"
[syn: cow, moo-cow]