In baseball terminology, a brushback is a pitch, often used for intimidation purposes, thrown near a batter's head. A pitcher will throw a brushback if he thinks the batter is crowding over the plate, is being a showboat, or for retaliation.

Brushbacks can result in warnings, ejections, or even bench-clearing brawls.

The safeguarding of Earth and other worlds from biological cross-contamination (i.e. billionaires with too much time on their hands).

Also known as “planetary protection.” Planetary protection / quarantine “reflects both the unknown nature of the space environment and the desire of the scientific community to preserve the pristine nature of celestial bodies until they can be studied in detail.”

There are two types of interplanetary contamination. Forward contamination is the transfer of viable organisms from Earth to another celestial body. Back contamination is the transfer of extraterrestrial organisms, if such exist, back to the Earth's biosphere.

to disagree, to overrule, to brush aside.

The act of accidentally brushing your penis against the toilet seat / bowl when sitting down.

To refute, reject, deny or disagree with a position or argument made by another to advance their agenda. Cross-reference,
brushback, e.g., the brushback pitch in baseball to reclaim the corners of the strike zone from an encroaching batter.