Cutting in line; inserting oneself out of turn (also used as the verb 'to cut in').
バスの列で横入りするのはマナー違反だ。
Cutting in line at the bus stop is bad manners.
会議中に他人の発言に横入りして意見を述べた。
He cut in on someone else's remark during the meeting to give his opinion.
店員が気付かずに、後から来た人がレジに横入りしてしまった。
The clerk didn't notice and someone who arrived later cut in at the register.