What does "a storm in a teacup" mean — and why is it funny?

informal, British

Meaning

A great deal of anger or worry about something that is really very small and unimportant.

Where it comes from

A British phrase from the 1800s — Americans say 'a tempest in a teapot' — shrinking a violent storm down to the size of teatime.

Why it is funny

The comedy is the mismatch of scale. It pictures thunder, wind and crashing waves all raging inside a dainty teacup, which is exactly how an overblown fuss looks to everyone not involved in it.

Used in a sentence

"The whole argument about the seating plan was a storm in a teacup."