露马脚 [- mǎ jiǎo]give oneself away; betray one's identity; let the cat out of the bag; reveal the true state of affairs; show one's hoof; show one's true colours:
•¶ 这个小偷因大量花钱而露了马脚。 The thief gave himself away by spending so much money.
•¶ 他无意中露出了马脚。 He betrayed himself without meaning to.