Green’s Dictionary of Slang

mother’s joy n.

(Aus.) a boy.

1905		Duke Tritton’s Letter n.p.: I hopped in. We drove about ten miles to his place and he introduced me to his Cheese and Kisses and four Tin Lids, two Mother’s Joys and two Twist and Twirls.
2002		Pete’s Aussie Sl. Home Page 🌐 mother’s joy: a boy.