Sjaak

This Dutch nickname for Jacob is derived from the French form Jacques.

Sjaak is not a diminutive of Jacob but rather a short form of Dutch Jakob or an adaptation of French Jacques. The last theory sounds more likely to me because the initial sj- would be an attempt the mirror the French zh- sound. In some cases, the name Sjaak can also be derived from Izaäk, the Dutch version of Isaac. [noted -ed]