Lupe is stealing this song from Kanye like a ballplayer steals a base. Lupe has the 3rd verse on this song, this song is the third track, and he is the third person to be heard on the album. (Kanye is first, Adam Levine is second)
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If you want to get technical, people would specifically refer to this line as a simile, as it is a type of metaphor; see MC Lyte’s line in Lyte as a Rock for details.
However, what Lupe’s really saying is “I think I like her like I met her ‘fore” (as in, “I met her before”), which is deliberately pronounced like “metaphor” here.
In addition, Lupe as a rapper loves to work with metaphors. and is a master at it. By saying he likes her like a metaphor, he shows his love for both, metaphors and the girl.
Lu’s also teasing us with the true meaning of this song: he likes “her like a metaphor” because she is, in fact, a metaphor for hip-hop. He mentions she’s “hard to get”, because as you can see people have a hard time picking up on it.