pitter-patter of little feet

Example: “Now that they've painted the spare room yellow, the family is waiting for the pitter-patter of little feet.”

Origin: Two separate things are at work. "Pitter-patter" is ablaut reduplication — a repeated word with the vowel changed — and it follows a rule English speakers apply without ever being taught it: the high front vowel comes first, which is why tick-tock, zigzag, and flip-flop are fixed in that order and sound wrong reversed. Attested from the 1500s, it imitates light, rapid tapping, originally of rain quite as much as of footsteps. The sentimental use for a coming child is Victorian, and Longfellow's poem "The Children's Hour" (1860) helped fix it, opening with the sound of little feet heard from the room overhead. The phrase now works almost entirely as a euphemism for pregnancy, announcing a baby by way of a sound it will not actually make for a good while yet.