長男
ちょうなん · chounan · JLPT N2 · noun
son
長 (chief, eldest) + 男 (male) = the chief male: the firstborn son
長 means chief or head, and 男 means man or male. Together they name the first son in the family line, the one who ranks at the head of the sons.
男 — field 田 + strength/plow 力
男 is a compound ideograph combining 田 (field) above 力 (strength, originally a picture of a plow). It depicted the one who works the fields with a plow, i.e. a male laborer, and by extension came to mean 'man' or 'male' in general.