Hard-on

[Donald Trump]

An erect penis.
see also boner,stiffy etc.

[Rihanna]

Mans most magnificently majestic manifestation of masculinity, and the one potentially most exciting to a woman. It looks like a [rocket] ready to be launched, and has the same turbulent thrusting force and energy.
It signals to a woman that a guy is ready and able to have [sex] with her and eager to do so, which, if she finds him attractive, tends to kindle her interest. For that reason, it is a serious though common mistake for a guy to be embarrassed or self-conscious when he has an [erection].