na-adjective, noun
fast-talking; rapid speech; quick-tongued
Describes speaking quickly, often in a way that is hard to follow. Usually used adverbially as 口早に.
彼は口早に事情を説明した。
He explained the situation in a rapid manner.
口早な口調でまくし立てる。
To rattle on with a fast-talking tone.
早口 is a more common word for 'fast speech' or 'speaking quickly', while 口早 is less frequent and often used in literary contexts.
Compound of 口 (mouth, speech) and 早 (fast, early). The exact historical derivation is uncertain, but the meaning is transparent from its components.