child born in the first half of the year
Refers to a child born between January and June, who is older relative to classmates in the Japanese school year (April–March). The term implies an advantage in physical and academic development compared to children born later in the year.
Children born in the first half of the year often have a larger build among their peers.
Because he was born early in the year, he has an advantage in sports.
Compound of 年 (とし, 'year') and 強い (つよい, 'strong'). The reading づよい is a rendaku form of つよい. The term likely arose from the observation that children born earlier in the school year tend to be more physically and academically advanced.