baby-sit definition by American Heritage Dictionary
verbba·by-sat also ba·by·sat , ba·by-sit·ting also ba·by·sit·ing, ba·by-sits also ba·by·sits verb, intransitive
To take care of a child or children in the absence of a parent or guardian.
To take care of or watch over someone or something needing attention or guidance.
To provide care for (a child) in the absence of a parent or guardian.
To watch over or tend: baby-sat the neighbor's plants for a week.
Word History: The verb baby-sit is of interest to parents, children, and linguists. It is interesting to the last group because it illustrates one of two types of the linguistic process called back-formation. The first type is based on misunderstanding, as in the case of our word pea. In Middle English the ancestor of pea was pese or pease, forms that functioned as both singular and plural. In other words, the s was part of the word, not a plural ending. But around the beginning of the 17th century people began to interpret the sound represented by s as a plural ending, and a new singular, spelled pea in Modern English, was developed. In the second type of back-formation, as seen in the case of baby-sit, first recorded in 1947, and babysitter, first recorded in 1937, no misunderstanding is involved. The agent noun babysitter with its -er suffix could have been derived from the verb baby-sit, as diver was from dive, but the evidence shows that the pattern was reversed, and the agent noun preceded the verb from which it would normally have been derived.