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It turns out that it comes from the English reserve. Note that the final /v/ in English becomes /b/ in Malay, just like in arkib ('archive').
What contributed to my failure to identify the word as coming from reserve is the omission of the second 'r'. If arkib keeps the 'r' from English, why does rizab not?
My colleague, Adrian Clynes, suggests it is because 'r' is fine as a part of a medial cluster, but not as part of a final cluster. So maybe that is the explanation.
One other thing about rizab: I showed it to some Bruneian students and colleagues, and none of them knew what it meant. Maybe it only exists in Malaysian Malay, not in Brunei.