A silly or foolish person. Now it's described for a person that simps for a girl on the internet by sending love letters/donating money. In 2020 the word simp still has some simmilarities to before like a person is silly/foolish by thinking he will marry an E-Girl.
Someone who gets weak in the knees at the ideal of another person; fantasizes about the relationship, perhaps hopes for more, but ultimately does not act upon their affections unless granted express permission.
John is such a simp, he'll hold onto any word she says to him and keep it in his pocket like a reminder.