榆木疙瘩

Explanation: Hard elm roots. Metaphor thought stubborn. 　　Source: Gloria "health team leader," everyone is the head of feudal superstition, the same as Elm knots, three axes five axes do not split open. " 　　Examples: Luo Xingang old summer? Do you know? "Summer is more than just knowledge! Is my father, a typical ~ head!" ★ "Xinhua Digest" 1981 (2) Thesaurus:  pimple