This is the 8046th most frequent Japanese word.
孟
"孟" means "chief," "eldest," or "first in rank."
In this sentence, '孟' is used as a prefix to a season name, "孟夏" (Mengxia), meaning "early summer."
孟夏は一年の中での暑さの始まりです。
Mengxia is the beginning of heat in a year.
Here, '孟' is a part of the name '孟子' (Mencius), referring to an individual whose name contains the character '孟.'
孟子は中国の歴史における有名な哲学者です。
Mencius is a famous philosopher in Chinese history.
'孟' in this sentence is utilized as a proper noun or name, possibly referring to a place or group associated with the character '孟.'