The story
From an incident reported in 1960 by J. Berko and R. Brown. A child referred to his inflatable plastic fish as a fis, but rejected adults' statements that used the same mispronunciation.
Etymology from Wiktionary, CC BY-SA 4.0.
fis phenomenon
Today it means: The observation that a child's perception of phonemes occurs earlier than the ability to produce them. Full definition →