What does “itsy bitsy” mean?
It means very, very small. Merriam-Webster lists “itsy-bitsy” with “itty-bitty”, meaning extremely small, and says the word probably comes from baby talk for “little bit”.
How to explain it to a child
Hold your finger and thumb almost touching and say, “Itsy bitsy means this small.” Then go looking for itsy bitsy things together: a crumb, a seed, a ladybug. “Incy wincy” and “eensy weensy” mean the same thing in the song.
Why it sounds so good in a rhyme
The two halves nearly match, so the word is fun to say and easy to remember: it-sy, bit-sy. Let your child invent one of their own: a wiggly-jiggly spider, or a teeny-weeny one.
In Spanish and Portuguese
Spanish speakers could say “pequeñita” for a very small spider, and Portuguese speakers could say “pequenina”. Both mean the same thing as “itsy bitsy”, though not with quite the same sound play.