Expressions (phrases, clauses, etc.), Ichidan verb
to interrupt; to butt in
ふたりでまじめな話をしているんだから、君、無責任な横槍を入れないでほしいんだけど。Hey, we're trying to have a serious discussion here so I'd appreciate it if you wouldn't butt in so irresponsibly.
Also written 横やりを入れる【よこやりをいれる】