This line can be interpreted a few different ways:
In the slough of events going on in Big Sean’s life, maybe his girl found out that he has cheated on her with a girl named Lisa. Sean changes the intonation of his voice when he says Lisa (meaning he says Lisa more like he is asking a question). In this situation, Sean’s girl has asked about a girl named Lisa and he is denying that anything has happened by stating “Lisa? ewww”
Sean might have penetrated a girl named Lisa in a grotesque manner when he had had too much to drink and he doesn't
Realize how disgusting his actions were until now.
Possibly a reference to the girl he’s talking about in Mona Lisa.
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