Definitions

1. adjective

being the only son or the only male child of a family.

“He is the sonly child in a family of three children.”

2. noun

a male child, especially the only male child in a family.

“He is the sonly of the family and has a very special role.”

3. noun

the only male child of a family, especially in a situation where there are no other sons.

“The sonly is the heir to the family business.”