the word 'thunder' descends directly from the old english þunor, which is related to the old norse þórr (the god thor, literally 'thunder') and the dutch donder. these all trace back to the proto-germanic þunraz, itself derived from the proto-indo-european root (s)tenh₂-, meaning 'to thunder' or 'to roar'. this ancient root is onomatopoeic, imitating the deep, rumbling sound of a storm. the word's journey from a sound-imitative proto-indo-european root through germanic languages into english illustrates how the name for a powerful natural phenomenon was personified in mythology (thor) while retaining its core descriptive meaning for the sound itself.

this word is a simplex form. it is a native english word inherited from old english þunor, which is of germanic origin.