IPA: /spɒnˈteɪnəsli/
KK: /spɑnˈteɪnəsli/
In a way that happens naturally or without being planned; occurring without external cause or effort.
The children laughed spontaneously when they saw the puppy playing in the park.
Spontaneously → It is formed from "spontaneus" (from Latin, meaning occurring naturally) and the suffix "-ly" (meaning in a manner). The word "spontaneously" means occurring in a natural manner without external influence.
Think of something happening 'naturally' ('spontaneus') and 'in a manner' ('-ly') — that's why spontaneously means occurring naturally.