Why isn't it called adultnapped ?

WORD HISTORY Appropriately enough, kidnapper seems to have originated among those who perpetrate this crime. We know this because kid and napper, the two parts of the compound, were slang of the sort that criminals used. Kid, which still has an informal air, was considered low slang when kidnapper was formed, and napper is obsolete slang for a thief, coming from the verb nap, “to steal.” Nap is possibly a variant of nab, which also still has a slangy ring. In 1678, the year in which the word is first recorded, kidnappers plied their trade to secure laborers for plantations in colonies such as the ones in North America. The term later took on the broader sense that it has today. The verb kidnap is recorded later (1682) than the noun and so is possibly a back-formation, that is, people may have assumed that a kidnapper kidnaps. We are all kids at heart

kid is a baby goat

original meaning

stealing goats