Meaning deny? Hmm can't find any connection towards the present meaning. Croats have podrijeti as in 'confer origin of', and also podrijetlo (porijeklo) sharing the same etymology according to Skok, порећи would rather be a newer than an older construct.
rěklo, rijetlo, ricati – I would interpret as source/sourcing, thus the construct po (negation) + ricati, builds poricati = to negate the source, namely deny something.
Pod-rěklo as well as Pod-rijetlo – however builds the construct 'from which source'. In Serbian it is common to lose the unpronounced letters, thus Po(d)rěklo > Porěklo