What does "a curtain twitcher" mean — and why is it funny?

informal, British, humorous

Meaning

A nosy person who secretly watches their neighbours from inside their house.

Where it comes from

A British phrase. The curtain 'twitches' as the watcher pulls it aside a little to peer out, then lets it fall the moment they are noticed.

Why it is funny

The humor is the tiny, guilty movement it is named for. The whole of someone's nosiness is captured in one small twitch of fabric — the curtain that gives the spy away every time.

Used in a sentence

"The street's resident curtain twitcher knows exactly who came home late."