I got inside her mental, chicks that I been through, it's simple
Steelo regarded himself as an indigo child who can see through people and tap into their being. He can see everything about this girl: inside her mind, thoughts, and emotions. For STEEZ however, it is not a difficult task to perform, unlike using psychokinesis to move a spoon.
Your “Mental” is your mind’s state of being, where your “head” is, if you will, and both these terms are slanx for sex. STEEZ might also have been using this as a double entendre to continue from the last line, saying “Miss Applebum” gave him some “head”, cause he’s slick with women like that, “it’s simple”.
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