It is a lunfardism that is used to describe someone as "dumb, naïve", who is also said to be "a fish", so 'piscuí' (with its phonetic resemblance) is also a familiar way of calling the fish. It is assumed that its etymology is precisely the reverse, that from 'piscuito' (a childish voice for fish) appears 'puscuí' as an apocope and as a euphemistic adjective for who is or behaves like a fish ("unwary"); but there is the word in English pipsqueak that is very similar in pronunciation and qualifies the "insignificant person", so it can be assumed as an origin although I do not know that it has ever been popular in the Río de la Plata to end up dissimilated in the lunfardo. For some mistake see piscuís (bird).
