露马脚 is an idiom meaning 'to give oneself away' or 'to reveal one's hidden secret/true colors'. The literal image is of a mythical creature that hides its cloven hoof but eventually exposes it. It functions like a separable verb (离合词): you can insert 了, 出, or a time expression between 露 and 马脚, e.g. 露了马脚 or 露出马脚. The pronunciation of 露 in this idiom is lòu (not lù).