I have seen pronk used as a verb to mean leaping through the air, as an antelope does. However, in a new word book I have, the first definition of it is a weak or foolish person. It says that the origin is uncertain, but may come from the Dutch word meaning fop. Has anyone ever seen it used as a weak or foolish person?

Never. I remember the "antelope" meaning from nature TV-shows, for the distinctive, way that antelopes bound when running.

I've never seen it but it sounds South African/Afrikaans.

Arnie, yes, my source says that the use with regard to "leaping" may be South African. Perhaps one use is from a South African derivation, while the other is from the Dutch derivation.

Afrikaans is a variant of Dutch, so that might be what your source is thinking of.

Arnie, my source "Weird and Wonderful Words" by Erin McKean, says, "'Pronk' is also a verb used in South Africa of a springbok's leap in the air, especially as an alarm signal". It first said that the "weak or foolish person" definition may have come from a Dutch word meaning "fop". It did say the origin is uncertain.