off the top of your headadvadverb: Describes a verb, adjective, adverb, or clause--for example, "come quickly," "very rare," "happening now," "fall down."
informal, figurative (spontaneously, improvising)
不假思索地 bù jiǎ sī suǒ de
信口 xìn kǒu
Off the top of my head, I can't remember the name of that actor.