Green’s Dictionary of Slang

dop n.2

[Du. dop, a husk, a shell, used generally for any bowl-shaped or spherical object]
(S.Afr.)

1. one’s head or brain.

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[SA] informant in DSAE (1996).

2. a motorcycle crash helmet.

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J. Hobbs Blossom 57: Dumbo [...] scorns to wear a crash helmet (what he calls a dop) [DSAE].