Green’s Dictionary of Slang

donder n.

(S.Afr.) ‘a blighter, a bastard’ (DSAE).

1959		H. Bloom Transvaal Episode 50: When he did tell us it was too late. The donders had run away.
1962		A. La Guma Walk in the Night (1968) 82: Get back, you donders. You’ll get hurt.

In compounds

donderkop (n.)

(S.Afr.) a fool.

1897		E. Glanville Tales from the Veld 120: ‘What do they whistle for?’ [...] ‘They whistle for the rain, you donder-kop.’.
1913		C. Pettman Africanderisms 149: Donderkop, A term of abuse equivalent to ‘blockhead’.