Stupid, so dummy
Gambino implies multiple meanings to this line.
1) That he’s stupid (a dummy) for pretending to like a girl he has no genuine feelings for.
2) That the girl is foolish for going along with it.
This is also probably a reference to 30 Rock (another NBC show that Glover used to write for) in which Liz Lemon’s boyfriend often calls her dummy. Lemon’s boyfriend was not very kind to her, so this could be him saying he wasn’t very kind to any of these girls.
“so dummy” could also possibly be a double entendre linked with the next line. When Gambino says the “wrong thing” i.e. mispronounces sodomy as “so dummy,” the wrong girl’s cum running.
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