Literally 'lip skin'. Almost always used figuratively in set phrases: 耍嘴皮子 'to be a smooth talker', 动嘴皮子 'to talk (without acting)', 嘴皮子薄/厚 'thin/thick lips' (figuratively, slick/loose-lipped). Distinctly colloquial.
register
Casual and slightly disparaging - implies the speaker is all talk and no action.
