Gain admittance, as to a party or concert, without being invited or without paying. For example, The concert was outdoors, but heavy security prevented anyone from crashing the gate. This term originally applied to persons getting through the gate at sports events without buying tickets. By the 1920s it was extended to being an uninvited guest at other gatherings and had given rise to the noun gatecrasher for one who did so. [Early 1900s]

Also, give someone the brush off or the gate or the old heave-ho . Break off relations with someone, oust someone, snub or jilt someone, especially a lover. For example, John was really upset when Mary gave him the air, or His old friends gave him the brush off, or Mary cried and cried when he gave her the gate, or The company gave him the old heave-ho after only a month. In the first expression, which dates from about 1920, giving air presumably alludes to being blown out. The second, from the first half of the 1900s, alludes to brushing away dust or lint. The third, from about 1900, uses gate in the sense of "an exit." The fourth alludes to the act of heaving a person out, and is sometimes used to mean "to fire someone from a job" (see get the ax). All these are colloquialisms, and all have variations using get, get the air (etc.), meaning "to be snubbed or told to leave," as in After he got the brush off, he didn't know what to do.