脑壳 (nǎo ké) is a colloquial noun that literally means 'skull/cranium', but is very commonly used in everyday speech (especially in Southwestern Mandarin) to simply mean 'head'. It can also be used humorously to refer to a person's general appearance or looks, much like calling someone's 'head' funny. As an HSK 7 word, it's useful for understanding regional or informal speech, but in standard writing you'd usually use 头 for 'head' and 颅骨 for 'skull'.