What does "pushing up daisies" mean — and why is it funny?

informal, humorous

Meaning

Dead and buried.

Where it comes from

An English phrase popularised around the time of the First World War. The image is of a body in the ground, nourishing the flowers that grow above it.

Why it is funny

The humor is the gentle, almost cheerful spin it puts on death. Instead of anything grim, it pictures the deceased quietly gardening from below, helpfully pushing daisies up into the sunshine.

Used in a sentence

"By the time that oak tree is full-grown, I'll be pushing up daisies."