The act of sexually arousing a female. As her vagina becomes wet, her basement floods.

Arousing a woman to the point of her getting wet, and then leaving without any sort of intercourse/sexual relief.

Made famous from the drag queen Ginger Minge on Ru Paul's Drag Race Season 7, flood the basement refers to moistening of the vagina prior to or during sexual intercourse. The joke here is that (as a woman in drag) she is so turned on by said person that instead of being moist, she flooded her pants with twat juice.

Seeing someone attractive and it makes you feel horny

getting an erection.

When something excites you to the extent of achieving, moist-ness in your genitals.

Flooding basements means to turn a girl on and make her wet.