When baby sitters first started baby-sitting, we had no way to talk about what they were doing. That's because at first, all we had was a noun – there was no verb to speak of.
According to the Oxford English Dictionary, the noun "baby sitter" was first recorded in 1937. The verb "baby-sit" didn’t come along until 1946.
Linguists call this a back-formation.
A back-formation is when a new word is formed by removing "actual or supposed affixes." In this case, the "er" suffix is removed from the noun "baby sitter" to form the verb "baby-sit."
There are other back formations in the history of English. For instance, we had "editor" before we had "edit,” we had "television" before we had "televise," and we had “diagnosis” before we had “diagnose.”
Now that the verb has been around for at least 70 years, babies aren't the only things we find ourselves baby-sitting. We can baby-sit the tea kettle while we wait for it to whistle, or we can spend the whole weekend baby-sitting the television. Sometimes we even baby-sit adults.
You'll notice in this article that in the verb form, we use a hyphen, “baby-sit,” and in the noun form, “baby sitter,” we separate the two words. That's because we follow AP Stylebook guidelines.
If that’s not your preference, don’t worry – not everyone agrees. For instance, Merriam-Webster and lexicographer Bryan Garner both avoid the hyphen and simply squish the two words together for both the noun and the verb, “babysit" and babysitter” while the American Heritage Dictionary includes both the hyphen and squished-together versions.
What's your preference?