old boy

US: (ōld′ boi′ for 1, 2; ōld′ boi′ for 3; ōld′ boi′ for 4)

WordReference Random House Unabridged Dictionary of American English © 2025

old boy  (ōld′ boi′ for 1, 2; ōld′ boi′ for 3;
ōld′ boi′ for 4), 

1. Informal Terms (sometimes disparaging). an adult male, esp. a Southerner. Cf. good old boy. 
2. a lively elderly man.
3. British Terms [Chiefly Brit.] an alumnus, esp. of a boys' preparatory or public school.
4. British Terms [Chiefly Brit.] See old chap. 

• 1595–1605

Collins Concise English Dictionary © HarperCollins Publishers::

old boy noun

1. (sometimes capitals) a male ex-pupil of a school
2. a familiar name used to refer to a man
3. an old man