In the Isan Lao dialect, the guava is called mak sida (Thai: หมากสีดา), mak being a prefix for fruit names. Thus bak sida (Thai: บักสีดา), bak being a prefix when calling males, refers jokingly to a Westerner.
ผัก is the prefix for vegetable. I would have thought if you could actually speak Thai you would know this 555.