Today most people would refer to them as “method files”, but
“The .m extension originally stood for
“messages” when Objective-C was first
introduced, referring to a central
feature of Objective-C […]”
(from the book “Learn Objective-C on the Mac” by Mark Dalrymple and Scott Knaster, page 9)
EDIT: To satisfy an itch I emailed Brad Cox, the inventor of Objective-C, about the question and he answered with this single line:
“Because .o and .c were taken. Simple
as that.”
Here’s the email as visual proof:
