What does 冰山難靠 mean?

冰山難靠 (빙산난고) literally means, "An ice mountain (冰山) is hard (難) to rely on (靠)." Why? Because it does not last; it melts when the weather is hot. It is an old saying (古事成語 - 고사성어) from the Tang Dynasty period (618 - 907 A.D.)

When Yang Guozhong (楊國忠 - 양국충) was Head of the Legislative Bureau (右相 - 우상), during the Tang Dynasty, he had so much authority that he was compared to Mount Tai (泰山 - 태산), one of China's "five sacred mountains" (五嶽 - 오악). People hoping to enrich themselves gathered around him, but a man named Zhang Tuan (張彖 - 장단)  was not so easily convinced. The following is a 752 A.D. entry in the "Zizhi Tongjian" (資治通鑑 - 자치통감), which is where the old saying 冰山難靠 supposedly originated.

或(혹)勸(권)陝(섬)郡(군)進(진)士(사)張(장)彖(단)謁(알)國(국)忠(충)曰(왈), "見(견)之(지), 富(부)貴(귀)立(립)可(가)圖(도)."

Someone (或) advised (勸) a palace graduate from Shan Commandery (陝郡進士) [named] Zhang Tuan (張彖) to visit (謁) Guozhong (國忠), saying (曰), "[If you] meet (見) him (之),  riches (富) and honors (貴) immediately (立) can (可) be obtained (圖)."

彖(단)曰(왈), "君(군)輩(배)倚(의)楊(양)右(우)相(상)如(여)泰(태)山(산). 吾(오)以(이)為(위)冰(빙)山(산)耳(이). 若(약)皎(교)日(일)既(기)出(출), 君(군)輩(배)得(득)無(무)失(실)所(소)恃(시)乎(호)?"

Tuan (彖) said (曰), "You guys (君輩) rely on (倚) Legislative Head Yang (楊右相) as if (如) [he were] Mount Tai (泰山). I (吾) consider [him] (以為) an ice mountain (冰山) only (耳). If (若) the bright (皎) sun (日) is out (既出), [won't] you guys (君輩) get (得) nothing (無) [and] lose (失) what you rely on (所恃乎)?

Notes

進士 - palace graduate; someone who has passed the highest level civil service exam
陝郡 - Shan Commandery
既出 - already (既) came out (出)
The entry was recorded in 752 A.D. - 唐紀 (당기) 玄宗天寶十一年 (현종천보 12년)