Curmudgeon
Etymology
Recorded from the late 1500s, and its origin is genuinely unknown. It is famous in dictionary lore because Samuel Johnson’s 1755 dictionary passed on a false etymology — that it came from French "coeur méchant" ("evil heart") — on the word of an anonymous correspondent. Later lexicographers repeated the guess as fact, making curmudgeon a textbook warning against inventing origins.
noun · /kur-MUJ-un/
A bad-tempered, cantankerous person, especially an older one.
What it names
An affectionate-to-scornful name for the reliably grumpy — the person whose crankiness is practically a personality and, sometimes, a charm.
Example
The old curmudgeon complained about the new coffee machine for a month, then quietly used it every day.
Related idea
A misanthrope dislikes humanity on principle; a curmudgeon is more of a habitual grumbler, often soft underneath.