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original info by Drievis Johan Jonker © 2014 - citations pending. see also -> http://en.wikipedia.org/wiki/Jonkheer
The Jonker surname was originally a noble 'designation' or generic 'noble title', not a full noble 'title' (like Duke, Earl etc) It was used for, or given to, members of the court who were to be knighted or titled at some point in the future, and generally for those of noble birth.
The meaning is literally: Young Gentleman.
It has origins in central and Western Europe but more specifically, the Netherlands and Prussia (Germany). It later became a 'Surname' used by, and given to, all the family of a "Jonkheer".
Variations are: Jonkers Jonckheere Jonghheer Joncker Juncker Junckher Junker Junkers Junkher Junckherre Junckheere Jonckheer Jonkheer Junkheer Junkers Junckers Junkhers... and more.
The Dutch and Germans would also give the (title) designation to any untitled nobleman from any country under their jurisdiction.
Therefore we can assume that if your surname is 'Jonker' then you have nobility in your bloodline somewhere, and most of the time they were from Europe.... with a few exception like 'Jonker from Macassar'. (Dutch Cape Colony circa 1709)