expression, suru verb (irregular)
address without honorific suffix; call by name only
Used when calling or referring to someone without adding a title or honorific suffix such as さん, くん, ちゃん, or 様. Often considered impolite or overly familiar unless the relationship is very close.
先輩を呼び捨てにするのは失礼だ。
It's rude to address a senior colleague without an honorific.
彼は親しい友達でも呼
He doesn't even call his close friends by name only.