"so 1995" and "so 1999" - mean that they were fashionable in 1995 and 1999 respectively, but aren't any more.
"most dudes just hit it and quit it" - referring to the fact that most guys just sleep with a lady and then leave her.
"most girls just spit it" - referring to girls who refuse to swallow after performing fellatio (oral sex), often seen as a sign that they are unwilling to submit to their partner sexually.
"not a lady if you're always on your knees" - referring to girls who perform fellatio in order to get guys to like them
"shouldn't have to beg a gentleman" - referring to the fact that a real gentleman would not have to receive sexual favours in order to get him to do something
"you're beautiful" - she is telling the girl that she is beautiful, presumably the girl is losing her self-esteem because of so many guys mistreating her.
"if you can't see, then maybe he's not meant to be" - referring to the fact that guys who manipulate can only have effective relationships with women who know full well they are trying to manipulate them and play them at their own game, but women who can't see it won't ever make it work.
"he already had the milk, so why would he go buy the cow" - referring to guys who have got what they want from a girl (milk, in this case a metaphor for sex) so he doesn't need to buy the cow (make a commitment)
"let you taste my rainbow" - referring to allowing a guy to perform oral sex on her, an act which a lot of women reserve for the person they are most intimate with
"I need to find my Obama" - referring to Barack Obama, seen as a gentleman
"Jonas Brother" - a member of the band The Jonas Brothers
"Kanye, he ain't a gentleman" - referring to Kanye West
"gentleman, 9 to 5" - referring to the fact that men who have "9 to 5" jobs are desirable, perhaps because most well paid jobs are 9 to 5 on paper, or perhaps because a guy who works these hours is free in the evenings for dates.
"go go go 90s" - she is celebrating the 1990s when gentlemen were easier to come by.
"so 1995" and "so 1999" - mean that they were fashionable in 1995 and 1999 respectively, but aren't any more. "most dudes just hit it and quit it" - referring to the fact that most guys just sleep with a lady and then leave her. "most girls just spit it" - referring to girls who refuse to swallow after performing fellatio (oral sex), often seen as a sign that they are unwilling to submit to their partner sexually. "not a lady if you're always on your knees" - referring to girls who perform fellatio in order to get guys to like them "shouldn't have to beg a gentleman" - referring to the fact that a real gentleman would not have to receive sexual favours in order to get him to do something "you're beautiful" - she is telling the girl that she is beautiful, presumably the girl is losing her self-esteem because of so many guys mistreating her. "if you can't see, then maybe he's not meant to be" - referring to the fact that guys who manipulate can only have effective relationships with women who know full well they are trying to manipulate them and play them at their own game, but women who can't see it won't ever make it work. "he already had the milk, so why would he go buy the cow" - referring to guys who have got what they want from a girl (milk, in this case a metaphor for sex) so he doesn't need to buy the cow (make a commitment) "let you taste my rainbow" - referring to allowing a guy to perform oral sex on her, an act which a lot of women reserve for the person they are most intimate with "I need to find my Obama" - referring to Barack Obama, seen as a gentleman "Jonas Brother" - a member of the band The Jonas Brothers "Kanye, he ain't a gentleman" - referring to Kanye West "gentleman, 9 to 5" - referring to the fact that men who have "9 to 5" jobs are desirable, perhaps because most well paid jobs are 9 to 5 on paper, or perhaps because a guy who works these hours is free in the evenings for dates. "go go go 90s" - she is celebrating the 1990s when gentlemen were easier to come by.