I am looking for something similar to IBM Watson but open source.
I am not aware of any open-source products that can readily replace IBM Watson, but the following projects could be of interest to you in that regard:
Try UIMA and GATE, both of them are open source.
Watson Content Analytics implements UIMA framework, according to this: Open, scalable analytics pipeline | IBM Watson Content Analytics 3.5.0
UIMA takes care of the management of NLP pipeline, but the intelligence is actually comes from ‘annotators’.
You can rely on annotators from projects like GATE, Apache OpenNLP, Stanford CoreNLP, etc. But since these projects are not part of UIMA, they use their own document/sentence representation, so a UIMA wrapper is needed for conversion. GATE provided a detailed documentation for this. Combining GATE and UIMA | GATE
Have a look at the ChatScript project, whose description is
ChatScript is a "next Generation" chatbot engine, based on the one that powered Suzette, that won the 2010 Loebner Competition. ChatScript has many advanced features and capabilities that, when properly utilitized, permit extremely clever bots to be programmed. There is also a potentially useful ontology of nouns, verbs, adjectives, and adverbs for understanding meaning.